Passive containment cooling system for a nuclear reactor

ABSTRACT

A nuclear plant includes a nuclear reactor, a containment structure that at least partially defines a containment environment of the nuclear reactor, and a passive containment cooling system that causes coolant fluid to flow downwards from a coolant reservoir to a bottom of a coolant channel coupled to the containment structure and rise through the coolant channel toward the coolant reservoir due to absorbing heat from the nuclear reactor. A check valve assembly, in fluid communication with the coolant reservoir, selectively enables one-way flow of a containment fluid from the containment environment to the coolant reservoir, based on a pressure at an inlet being equal to or greater than a threshold magnitude. A fusible plug, in fluid communication with the coolant reservoir at a bottom vertical depth below the bottom of the coolant reservoir, enables coolant fluid to flow into the containment structure based on at least partially melting.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. application Ser. No.16/726,355, filed Dec. 24, 2019, the entire contents of which are herebyincorporated herein by reference.

BACKGROUND Field

Example embodiments described herein relate in general to nuclearreactors and in particular to providing passive cooling of a nuclearreactor containment.

Description of Related Art

Nuclear reactors may be configured to be cooled via heat transfer to oneor more coolant fluids circulated in or near the nuclear reactor. Suchheat transfer may be referred to herein as heat rejection by the nuclearreactor. Various coolant fluids may be utilized to remove heat from thenuclear reactor. A coolant fluid may be a fluid that includes one ormore various substances, including water, liquid metal, molten salt, agaseous substance, some combination thereof, etc.

In some nuclear plants, a nuclear reactor includes a containment system,also referred to herein as simply “containment,” for managing heatrejection by the nuclear reactor by facilitating circulation of acoolant fluid, such as water, to a point in the nuclear reactor wherethe coolant fluid absorbs heat rejected by the nuclear reactor, and theheated coolant fluid is then circulated to a heat return, or heat sink,where the heated coolant fluid may be cooled to release the absorbedheat. In some nuclear plants, the containment system may be impacted byheat rejection that exceeds the heat transfer capabilities of a powercoolant loop that is used to induce work, for example to generateelectricity. Accordingly, the containment system may utilize cooling tomanage containment system temperature or prevent the containment systemfrom exceeding its qualified temperature.

In some nuclear plants, a nuclear reactor may experience excursions oftemperature and/or pressure within a containment environment in whichthe nuclear reactor may be located. The temperature and/or pressurewithin the containment environment may be controlled to influenceperformance and/or integrity of the nuclear reactor. In some nuclearplants, such temperature and/or pressure control may be implementedthrough various control systems that manage pressure release and/orcooling of the containment environment. Such control systems may utilizecomputer-implemented functionality and/or operator-controlledfunctionality, which may thus consume electrical power, operatoroperations, some combination thereof, or the like. In addition, pressurecontrol within the containment environment may involve releasing fluidsfrom the containment environment.

SUMMARY

According to some example embodiments, a nuclear plant may include anuclear reactor, a containment structure having one or more innersurfaces at least partially defining a containment environment in whichthe nuclear reactor is located, and a passive containment coolingsystem. The passive containment cooling system may include a coolantreservoir configured to hold a coolant fluid, a coolant channel coupledto the containment structure such that the coolant channel extendsvertically from a coolant channel inlet at a bottom of the coolantchannel to a coolant channel outlet at a top of the coolant channel, anda coolant supply conduit extending downwards from an inlet of thecoolant supply conduit that is open to a lower region of the coolantreservoir. An outlet of the coolant supply conduit may be coupled to thecoolant channel inlet, such that the coolant supply conduit isconfigured to direct a flow of coolant fluid downwards out of the lowerregion of the coolant reservoir and into the bottom of the coolantchannel via the coolant channel inlet according to gravity, such thatthe coolant fluid rises through the coolant channel from the bottom ofthe coolant channel to the top of the coolant channel according to achange in coolant fluid buoyancy based on the coolant fluid absorbingheat rejected from the nuclear reactor in the containment environment.The passive containment cooling system may include a coolant returnconduit having an inlet coupled to the coolant channel outlet at the topof the coolant channel. The coolant return conduit may extend upwardsfrom the inlet of the coolant return conduit to an outlet of the coolantreturn conduit that is open to an upper region of the coolant reservoirthat is above the lower region of the coolant reservoir, such that thecoolant return conduit is configured to direct a flow of the coolantfluid to rise out of the top of the coolant channel via the coolantchannel outlet and into the upper region of the coolant reservoiraccording to increased buoyancy of the coolant fluid at the top of thecoolant channel over the buoyancy of the coolant fluid at the bottom ofthe coolant channel.

The passive containment cooling system may include a first check valveassembly at a first vertical depth below a top surface of coolant fluidin the coolant reservoir, the first check valve assembly in fluidcommunication with the coolant reservoir and with the containmentenvironment. The first check valve assembly may include one or morecheck valves coupled between a first check valve assembly inlet and afirst check valve assembly outlet. The first check valve assembly inletmay be in fluid communication with the coolant reservoir. The one ormore check valves may be configured to open in response to a pressure atan inlet of the one or more check valves being equal to or greater thana first threshold magnitude, the first threshold magnitude at leastpartially corresponding to a hydrostatic pressure of the coolant fluidat the check valve assembly outlet at the first vertical depth. Thefirst check valve assembly may be configured to selectively enableone-way flow of a containment fluid, from the containment environmentvia the first check valve assembly inlet to the coolant reservoir viathe first check valve assembly outlet, based on the one or more checkvalves opening in response to a pressure of the containment environmentat the first check valve assembly inlet at the first vertical depthbeing equal to or greater than the first threshold magnitude.

The first check valve assembly may extend through the containmentstructure and into the coolant channel at the first vertical depth, andthe first check valve assembly may be open to the coolant channel, suchthat the first check valve assembly is in fluid communication with thecoolant reservoir through the coolant channel. The first check valveassembly may be configured to selectively enable the one-way flow of thecontainment fluid, from the containment environment via the first checkvalve assembly inlet, to the coolant channel via the first check valveassembly outlet.

The first threshold magnitude may be greater than a referencehydrostatic pressure of the coolant fluid in the coolant channel at thefirst vertical depth below the bottom of the coolant reservoir thatresults from the coolant reservoir being filled to a reference reservoirdepth, such that the reference hydrostatic pressure of the coolant fluidin the coolant channel at the first vertical depth is equal to ahydrostatic pressure of the coolant fluid at a particular vertical depththat is a sum of the first vertical depth and the reference reservoirdepth.

The first check valve assembly may be configured to, subsequently toselectively enabling the one-way flow, inhibit the one-way flow of thecontainment fluid based on the one or more check valves closing inresponse to the pressure of the containment environment at the firstcheck valve assembly inlet being less than the first thresholdmagnitude.

The one or more check valves may include a series connection of aplurality of check valves between the first check valve assembly inletand the first check valve assembly outlet. Each check valve of theplurality of check valves may be configured to open in response to apressure at an inlet of the check valve being equal to or greater thanthe first threshold magnitude. The first check valve assembly may beconfigured to selectively enable the one-way flow based on all checkvalves of the series connection of the plurality of check valvesopening.

The one or more check valves may include a parallel connection of aplurality of sets of one or more check valves between the first checkvalve assembly inlet and one or more check valve assembly outlets. Eachcheck valve of the plurality of sets of one or more check valves may beconfigured to open in response to a pressure at an inlet of the checkvalve being equal to or greater than the first threshold magnitude. Thefirst check valve assembly may be configured to selectively enable theone-way flow based on any set of one or more check valves of theparallel connection of the plurality of sets of one or more checkvalves.

The first check valve assembly may include a burst disc coupled inseries with the one or more check valves. The burst disc may beconfigured to rupture in response to a pressure increase in thecontainment environment to a particular (or, alternatively,pre-determined) threshold (e.g., “set point”) pressure magnitude,thereby allowing the containment fluid pressure to reach the inlet ofthe first check valve assembly which allows containment fluid flow whenthe pressure at the inlet is equal to or greater than the firstthreshold magnitude.

The nuclear plant may further include a second check valve assembly at asecond vertical depth below the top surface of coolant fluid in thecoolant reservoir. The second check valve assembly may be in fluidcommunication with the coolant reservoir and with the containmentenvironment. The second vertical depth may be less than the firstvertical depth. The second check valve assembly may be configured toselectively enable one-way flow of the containment fluid, from thecontainment environment to the coolant reservoir, based on one or morecheck valves of the second check valve assembly opening in response to apressure of the containment environment at an inlet of the second checkvalve assembly being equal to or greater than a second thresholdmagnitude. The second threshold magnitude may at least partiallycorrespond to a hydrostatic pressure of the coolant fluid at an outletof the second check valve assembly at the second vertical depth.

The nuclear plant may further include a fusible plug in fluidcommunication with the coolant reservoir and with the containmentenvironment at a bottom vertical depth below the top surface of thecoolant fluid in the coolant reservoir. The bottom vertical depth may begreater than the first vertical depth, such that a hydrostatic pressureof the coolant fluid at the bottom vertical depth is greater than thehydrostatic pressure of the coolant fluid at the first check valveassembly outlet at the first vertical depth. The fusible plug may beconfigured to at least partially melt in response to a temperature inthe containment environment at an end of the fusible plug that is opento the containment environment being equal to or greater than athreshold temperature, such that the fusible plug exposes a flow conduitextending between the coolant reservoir into the containment environmentto at least partially flood the containment environment with at leastsome of the coolant fluid.

The first check valve assembly may be configured to, based onselectively enabling the one-way flow of the containment fluid inresponse to the pressure in the containment environment at the firstcheck valve assembly inlet being equal to or greater than the firstthreshold magnitude, maintain a pressure in the containment environmentat the bottom vertical depth at a magnitude that is less than thehydrostatic pressure of the coolant fluid at the bottom vertical depth,to enable flow of coolant fluid through the exposed flow conduit andinto the containment environment in response to the fusible plug atleast partially melting.

The first check valve assembly and the fusible plug may be collectivelyconfigured to enable circulation of coolant fluid within the containmentenvironment, from the coolant channel or other coolant routing pathwayat the bottom vertical depth to the containment environment via thefusible plug and from the containment environment at the first verticaldepth to the coolant channel or other coolant routing pathway via thefirst check valve assembly.

According to some example embodiments, a method for operating a passivecontainment cooling system for a nuclear reactor may include directing aflow of coolant fluid downwards out of a lower region of a coolantreservoir via a coolant supply conduit according to gravity to a bottomof a coolant channel that extends vertically along a containmentstructure that at least partially defines a containment environment inwhich the nuclear reactor is located, and causing the coolant fluid torise through the coolant channel from the bottom of the coolant channeltoward an upper region of the coolant reservoir via a top of the coolantchannel according to a change in coolant fluid buoyancy based on thecoolant fluid absorbing heat rejected from the nuclear reactor in thecontainment environment via at least the containment structure.

The method may include selectively enabling a one-way flow of acontainment fluid, from the containment environment to the coolantreservoir via a first check valve assembly at a first vertical depthbelow a top surface of coolant fluid in the coolant reservoir, the firstcheck valve assembly in fluid communication with the coolant reservoirand with the containment. The selectively enabling may be based on oneor more check valves of the first check valve assembly opening inresponse to a pressure at an inlet of the one or more check valves beingequal to or greater than a first threshold magnitude. The firstthreshold magnitude may at least partially correspond to a hydrostaticpressure of the coolant fluid at an outlet of the first check valveassembly at the first vertical depth.

The first threshold magnitude may be greater than a referencehydrostatic pressure of the coolant fluid in the coolant channel at thefirst vertical depth below the top surface of the coolant fluid in thecoolant reservoir that results from the coolant reservoir being filledto a reference reservoir depth, such that the reference hydrostaticpressure of the coolant fluid in the coolant channel at the firstvertical depth is equal to a hydrostatic pressure of the coolant fluidat a particular vertical depth that is a sum of the first vertical depthand the reference reservoir depth.

The method may further include inhibiting the one-way flow, subsequentlyto selectively enabling the one-way flow, based on the one or more checkvalves closing in response to the pressure of the containmentenvironment at an inlet of the first check valve assembly being lessthan the first threshold magnitude.

The one or more check valves may include a series connection of aplurality of check valves between an inlet of the first check valveassembly and the outlet of the first check valve assembly. Each checkvalve of the plurality of check valves may be configured to open inresponse to a pressure at an inlet of the check valve being equal to orgreater than the first threshold magnitude. The selectively enabling maybe based on all check valves of the series connection of the pluralityof check valves opening.

The one or more check valves may include a parallel connection of aplurality of sets of one or more check valves between an inlet of thefirst check valve assembly and one or more check valve assembly outlets.Each check valve of the plurality of sets of one or more check valvesmay be configured to open in response to a pressure at an inlet of thecheck valve being equal to or greater than the first thresholdmagnitude. The selectively enabling may be based on any set of one ormore check valves of the parallel connection of the plurality of sets ofone or more check valves.

The selectively enabling may be based on a burst disc coupled in serieswith the one or more check valves, for example, between the inlet of theone or more check valves and an inlet of the first check valve assembly,rupturing in response to a pressure at the inlet of the first checkvalve assembly at the first vertical depth being equal to or greaterthan the first threshold magnitude.

The method may further include directing at least a portion of thecoolant fluid at a bottom vertical depth below the top surface of thecoolant fluid in the coolant reservoir to flow into the containmentenvironment via an exposed flow conduit between the coolant reservoirand the containment environment at the bottom vertical depth to at leastpartially flood the containment environment, based on a fusible plug influid communication with the coolant reservoir and with the containmentenvironment, at the bottom vertical depth, at least partially melting toexpose the flow conduit in response to a temperature in the containmentenvironment at an end of the fusible plug that is open to thecontainment environment being equal to or greater than a thresholdtemperature.

The first check valve assembly, based on selectively enabling theone-way flow, may maintain a pressure in the containment environment atthe bottom vertical depth at a magnitude that is less than thehydrostatic pressure of the coolant fluid at the bottom vertical depth,to enable flow of coolant fluid through the exposed flow conduit andinto the containment environment in response to the fusible plug atleast partially melting.

The first check valve assembly and the fusible plug may collectivelyenable circulation of coolant fluid within the containment environment,from the coolant channel or other coolant routing pathway at the bottomvertical depth to the containment environment via the fusible plug andfrom the containment environment at the first vertical depth to thecoolant channel or other coolant routing pathway via the first checkvalve assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the non-limiting embodimentsherein may become more apparent upon review of the detailed descriptionin conjunction with the accompanying drawings. The accompanying drawingsare merely provided for illustrative purposes and should not beinterpreted to limit the scope of the claims. The accompanying drawingsare not to be considered as drawn to scale unless explicitly noted. Forpurposes of clarity, various dimensions of the drawings may have beenexaggerated.

FIG. 1 is a cross-sectional schematic side view of a nuclear plant thatincludes a passive containment cooling system that further includes acontainment venting system and a containment flooding system, accordingto some example embodiments.

FIGS. 2A-2C are expanded views of region A of FIG. 1 , according to someexample embodiments.

FIG. 3 is an expanded view of region A of FIG. 1 , according to someexample embodiments.

FIG. 4 is a flowchart that illustrates a method of operation of apassive containment cooling system, according to some exampleembodiments.

FIG. 5 is an expanded view of region B of FIG. 1 , according to someexample embodiments.

FIG. 6 is a perspective view of a passive containment cooling systemthat includes one or more coolant channels integrated into thecontainment structure, according to some example embodiments.

DETAILED DESCRIPTION

It should be understood that when an element or layer is referred to asbeing “on,” “connected to,” “coupled to,” or “covering” another elementor layer, it may be directly on, connected to, coupled to, or coveringthe other element or layer or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly connected to,” or “directly coupled to” another elementor layer, there are no intervening elements or layers present. Likenumbers refer to like elements throughout the specification. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

It should be understood that, although the terms first, second, third,etc. may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers, and/or sections should not be limited by these terms. Theseterms are only used to distinguish one element, component, region,layer, or section from another region, layer, or section. Thus, a firstelement, component, region, layer, or section discussed below could betermed a second element, component, region, layer, or section withoutdeparting from the teachings of example embodiments.

Spatially relative terms (e.g., “beneath,” “below,” “lower,” “above,”“upper,” and the like) may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It should be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” may encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The terminology used herein is for the purpose of describing variousembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an,” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“includes,” “including,” “comprises,” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Example embodiments are described herein with reference tocross-sectional illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of exampleembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, example embodiments should not be construed aslimited to the shapes of regions illustrated herein but are to includedeviations in shapes that result, for example, from manufacturing. Forexample, an implanted region illustrated as a rectangle will, typically,have rounded or curved features and/or a gradient of implantconcentration at its edges rather than a binary change from implanted tonon-implanted region. Likewise, a buried region formed by implantationmay result in some implantation in the region between the buried regionand the surface through which the implantation takes place. Thus, theregions illustrated in the figures are schematic in nature and theirshapes are not intended to illustrate the actual shape of a region of adevice and are not intended to limit the scope of example embodiments.

Example embodiments may be described with reference to acts and symbolicrepresentations of operations (e.g., in the form of flow charts, flowdiagrams, data flow diagrams, structure diagrams, block diagrams, etc.)that may be implemented in conjunction with units and/or devicesdiscussed in more detail below. Although discussed in a particularmanner, a function or operation specified in a specific block may beperformed differently from the flow specified in a flowchart, flowdiagram, etc. For example, functions or operations illustrated as beingperformed serially in two consecutive blocks may actually be performedsimultaneously, or in some cases be performed in reverse order.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, including those defined incommonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand will not be interpreted in an idealized or overly formal senseunless expressly so defined herein.

Although described with reference to specific examples and drawings,modifications, additions and substitutions of example embodiments may bevariously made according to the description by those of ordinary skillin the art. For example, the described techniques may be performed in anorder different with that of the methods described, and/or componentssuch as the described system, architecture, devices, circuit, and thelike, may be connected or combined to be different from theabove-described methods, or results may be appropriately achieved byother components or equivalents.

It will be understood that a “nuclear reactor” as described herein mayinclude any or all of the well-known components of a nuclear reactor,including a nuclear reactor core with or without nuclear fuelcomponents, control rods, or the like. It will be understood that anuclear reactor as described herein may include any type of nuclearreactor, including but not limited to a Boiling Water Reactor (BWR), aPressurized Water Reactor (PWR), a liquid metal cooled reactor, a MoltenSalt Reactor (MSR), or the like. As described herein, a nuclear reactormay include an Advanced Boiling Water Reactor (ABWR), an EconomicSimplified Boiling Water Reactor (ESBWR), a BWRX-300 reactor, or thelike.

It will be understood that a “coolant fluid” as described herein mayinclude any well-known coolant fluid that may be used in cooling anypart of a nuclear plant and/or nuclear reactor, including water, aliquid metal (e.g., liquid sodium), a gas (e.g., helium), a molten salt,any combination thereof, or the like. It will be understood that a“fluid” as described herein may include a gas, a liquid, or anycombination thereof.

The present disclosure relates to a unique passive containment coolingsystem that utilizes one or more coolant channels coupled to acontainment structure and extending vertically, from a coolant channelinlet at a bottom of the coolant channel to a coolant channel outlet ata top of the coolant channel, where the passive containment coolingsystem, also referred to herein as simply a “passive containment coolingsystem,” directs a coolant fluid to flow into the bottom of the coolantchannel via the coolant channel inlet such that the coolant fluid risesvertically through the coolant channel, from the bottom of the coolantchannel to a top of the coolant channel, according to a change incoolant fluid buoyancy based on the coolant fluid absorbing heatrejected from the nuclear reactor in the containment environment, wherethe coolant channel is coupled to the containment structure. The passivecontainment cooling system may supply the coolant fluid to the bottom ofthe coolant channel via the inlet thereof based on being directed toflow downwards (e.g., in the direction of gravitational acceleration)according to gravity from a coolant reservoir, via a coolant supplyconduit extending downwards from an inlet of the coolant supply conduitthat is open to a lower region of the coolant reservoir to an outlet ofthe coolant supply conduit that is coupled to the coolant channel inlet.Additionally, the passive containment cooling system may return thecoolant fluid to an upper region of the coolant reservoir due to theincreased buoyancy of the heated coolant fluid via a coolant returnconduit having an inlet coupled to the coolant channel outlet at the topof the coolant channel and extending upwards from the inlet of thecoolant return conduit to an outlet of the coolant return conduit thatis open to the upper region of the coolant reservoir that is above thelower region of the coolant reservoir to which the inlet of the coolantsupply conduit is open.

As a result, the passive containment cooling system may drivecirculation of coolant fluid upwards and out of the coolant channel andback into the coolant reservoir due to increased buoyancy due toabsorbing heat rejected from the nuclear reactor, and the rising coolantfluid may be displaced in the bottom of the coolant channel by coldercoolant fluid that flows downwards to the bottom of the coolant channelvia a separate coolant supply conduit according to gravity, therebyenabling removal of heat from the containment environment. Because thecolder coolant fluid is directed from a lower region of the coolantreservoir and the heated coolant fluid is directed into a higher, upperregion of the coolant reservoir, the heated coolant fluid may remainabove the colder coolant fluid in the coolant reservoir due to havingincreased buoyancy as a result of being heated by heat rejected from thenuclear reactor and thus being warmer than the cold coolant fluid, suchthat the coolant fluid that is directed to fall through the coolantsupply conduit to the bottom of the coolant channel may be colder thanthe heated coolant fluid that is returned to the coolant reservoir viathe coolant return conduit. Thus, it will be understood that the heatedcoolant fluid may be returned to the coolant reservoir via a coolantreturn conduit outlet that is open to the coolant reservoir at a greaterheight from a bottom of the coolant reservoir than the coolant supplyconduit inlet. Accordingly, the circulation of coolant through thepassive containment cooling system to remove heat rejected by thenuclear reactor to the coolant reservoir, where the coolant reservoirmay function as an at least temporary heat sink, may be “passive” inthat the circulation is not driven due to operation of a flow generatordevice, e.g., a pump, or based on intervention of an operator (e.g.,including a human and/or processing circuitry, such as a processorexecuting a program of instructions stored on a memory, that generatesan electrical control signal to control one or more devices), to induceor maintain a flow of coolant fluid. Accordingly, based on providing“passive” cooling of the nuclear reactor, the passive containmentcooling system may enable improved operational efficiency of the nuclearplant based on reducing energy consumption to operate the nuclear plantand improved safety by not relying upon operator or control systemintervention to control one or more devices to enable and/or control thecooling of the nuclear reactor.

The coolant channel may be any type of conduit, including a pipe that iscoupled (e.g., welded, bolted, secured through mechanical means, etc.)to a surface of a containment structure (e.g., an outer surface, aninner surface, an interior surface, any combination thereof, or thelike), a channel defined within an interior of a structure thatpartially or entirely defines the containment structure (e.g., anintegrated passive cooling containment structure), any combinationthereof, or the like.

The passive containment cooling system further may include one or morefirst check valve assemblies that enable passive control (e.g., controlthat is not controlled due to operator or control system intervention)of the pressure within the containment environment in which the nuclearreactor is located. The one or more first check valve assemblies are influid communication with both the containment environment and thecoolant reservoir and may selectively enable one-way flow (also referredto herein as performing “venting”) of containment fluid out of thecontainment environment and to the coolant reservoir, via one or morechannels and/or conduits to which the one or more first check valveassemblies are open and via which the one or more first check valveassemblies are in fluid communication with the coolant reservoir, basedon whether the pressure in the containment environment at the inlets ofthe one or more first check valve assemblies reaches (e.g., is equal toor greater than) a threshold pressure magnitude that corresponds to ahydrostatic pressure of the coolant fluid the coolant reservoir at theoutlets of the one or more first check valve assemblies, such that apressure gradient from the containment environment to the one or morecoolant channels or other pathway to the coolant reservoir through theone or more first check valve assemblies is ensured, thereby reducing orpreventing the risk of backflow through the one or more first checkvalve assemblies from the one or more coolant channels or other pathwayto the coolant reservoir into the containment environment.

As a result, the one or more first check valve assemblies mayselectively, based on actuation of one or more check valves includedtherein between a closed state and an open state, enable one-way flow ofa containment fluid from the containment environment to the coolantreservoir to relieve the pressure in the containment environment. Suchenabling of one-way flow of containment fluid to the coolant reservoirmay be referred to as “venting” of the containment environment. Thecontainment fluid may include one or more of a gas, liquid, solidmaterial entrained in a gas and/or liquid, any combination thereof, orthe like.

In some example embodiments, the first check valve assembly may extendthrough the containment structure and into a coolant channel at a depthbelow the reservoir, such that the first check valve assembly is open tothe coolant channel, is in fluid communication with the coolantreservoir through the coolant channel, and is configured to selectivelyenable the one-way flow from the containment environment to the coolantchannel at the depth, but example embodiments are not limited thereto.

The containment fluid may include radioactive material, and the one ormore first check valve assemblies may, based on the selectively enablingof one-way flow out of the containment environment, selectively “vent”the containment fluid into the coolant reservoir and/or the flow ofcoolant fluid in one or more coolant channels or other pathway to thecoolant reservoir, such that the containment fluid may be entrained inthe upwards flow of the coolant fluid to the top of the one or morecoolant channels or other pathway to the coolant reservoir and thus thecontainment fluid may be drawn into the coolant reservoir via the flowof the coolant fluid. As a result of being drawn into, and thus retainedin, the coolant reservoir based on being vented into the coolant fluidin the one or more coolant channels or other pathway to the coolantreservoir, the containment fluid may be restricted, at leasttemporarily, from being released to an exterior of the nuclear plant.The coolant reservoir, in addition to functioning as a heat sink forheat removed from the containment environment via the coolant fluid, mayfunction as a reservoir for radioactive materials included in thecontainment fluid. Additionally, containment fluid that includes a gas,such as water vapor (e.g., steam) may be condensed back into a liquidstate by the coolant fluid in the coolant channel and/or reservoir,thereby mitigating pressure buildup in the nuclear plant containment andreducing or preventing the need to vent gases to an atmosphere externalto the nuclear plant.

The one or more check valve assemblies may include one or more checkvalves that are configured to actuate, between open and closed states,based on whether a pressure at an inlet of the one or more check valvesreaches a threshold pressure. The one or more check valves may beconfigured to actuate open or closed (e.g., actuate to the open state orclosed state) based on the pressure at the inlet and thus without anyintervention by an operator (e.g., a human and/or processing circuitry)or control system to control the venting operation. Accordingly, theventing functionality provided by a check valve assembly may beunderstood to be “passive” at least by virtue of not operating based onoperator or control system intervention. As a result, containment may beimproved while also providing pressure release capability for thenuclear plant containment. It will be understood that the nuclear plant“containment” may encompass a structure that encompasses at least thecontainment environment, in which the nuclear reactor of the nuclearplant is located. It will be understood that “control system”intervention may refer to intervention by a control system that mayinclude one or more instances of processing circuitry, for example aprocessor executing a program of instructions stored on a memory, wherethe intervention performed by the control system may include, withoutlimitation, the control system generating an electrical signal, alsoreferred to a control signal, that is communicated (e.g., transmitted)to a device to cause the device or another, separate device to performan operation (e.g., actuate a valve, control a pump operation, etc.).

The passive containment cooling system may further include one or morefusible plugs in fluid communication with the coolant reservoir and withthe containment environment (e.g., based on the fusible plug(s)extending through the containment structure that at least partiallydefines the containment environment and into the coolant channel orother pathway) to the coolant reservoir at a depth that is below alowest depth below the coolant reservoir at which the one or more checkvalve assemblies are located, such that a hydrostatic pressure of thecoolant fluid in the coolant channel or other pathway to the coolantreservoir at the depth of the fusible plug in the coolant channel orother pathway to the coolant reservoir is greater than the greatesthydrostatic pressure of the coolant fluid in the coolant channel orother pathway to the coolant reservoir at the one or more check valveassembly outlet. The one or more fusible plugs, which may be anywell-known fusible plug, may be configured (e.g., based on including aparticular fusible alloy) to at least partially melt in response to atemperature in the containment environment at a portion of the fusibleplug that is open to the containment environment at least meeting athreshold temperature (e.g., a melting temperature of the particularfusible alloy), such that the fusible plug at least partially melts toexpose a flow conduit extending between the coolant reservoir and thecontainment environment via the fusible plug. As a result, at least someof the coolant fluid in the coolant channel or other pathway to thecoolant reservoir may at least partially flood the containmentenvironment, thereby providing temperature control in the containmentenvironment and aid in limiting nuclear reactor temperature.Additionally, the one or more first check valve assemblies may beconfigured to selectively actuate to ensure that the pressure in thecontainment environment at the fusible plug is less than the hydrostaticpressure of coolant fluid in the coolant channel or other pathway to thecoolant reservoir at the depth of the fusible plug, thereby ensuring apressure gradient from the coolant channel or other pathway to thecoolant reservoir into the containment environment when the temperaturein the containment environment at the fusible plug reaches the thresholdtemperature, thereby reducing or preventing the risk that coolant fluidmay not flow into the containment environment when the fusible plug atleast partially melts. The flooding of the containment environment mayprovide cooling of the nuclear reactor and/or the containmentenvironment and/or cooling of materials in the containment environment,including radioactive materials including, but not limited to fuelcontaining material (FCM), lava-like fuel containing material (LFCM),“corium” as the term is well-known to be understood in the nuclear powerindustry with regard to nuclear reactors, any combination thereof, orthe like.

The passive containment cooling system may be configured, based on theone or more first check valve assemblies being configured to actuate(and selectively enable the one-way flow out of the containmentenvironment) at a particular threshold pressure magnitude and thefusible plug being configured to at least partially melt at a particularthreshold temperature, to ensure that the fusible plug melts after theone or more first check valve assemblies have enabled the one-way flow,thereby enabling a flow path of fluid (e.g., coolant fluid) into thecontainment environment from the coolant channel or other pathway to thecoolant reservoir via the flow conduit exposed by the at least partiallymelted fusible plug, upwards through the containment environment to theone or more first check valve assemblies, and back into the coolantchannel from the containment environment via the one or more first checkvalve assemblies,

Based on providing a capability to at least partially flood thecontainment environment via at least partially melted fusible plugs,such flooding capability may be considered to be “passive” in that thecapability may be implemented without (e.g., independently of) operatoror control system intervention. Accordingly, cooling performance of thenuclear reactor in response to pressure and/or temperature excursions,and the containment of radioactive materials and the prevention ofrelease of said materials from the nuclear plant, may be improved.

It will be understood that, as described herein, a “check valve” may beinterchangeably referred to as a non-return valve, a reflux valve, aretention valve, a one-way valve, or the like and will be understood torefer to a valve that is configured to allow fluid (e.g., liquid and/orgas) to flow through the valve in only one direction (e.g., selectivelyenabling one-way flow) based on selectively actuating between a closedposition in which the one-way flow is inhibited and an open position inwhich the one-way flow is enabled. Check valves as described herein mayinclude any type of check valve that is well-known with regard toselectively enabling one-way fluid flow, including, without limitation,swing check valves, tilting disc check valves, clapper valves,stop-check valves, lift-check valves, in-line check valves, pneumaticnon-return valves, any combination thereof, or the like.

It will be understood that, as described herein, a “fusible plug” mayinclude any type of fusible plug that is configured to at leastpartially melt in response to at least a portion of the fusible plugbeing exposed to a temperature that reaches (e.g., is equal to orgreater than) a threshold temperature. For example, a fusible plug asdescribed herein may include a body cylinder (at least partiallycomprising a body material) that includes a conduit extending throughoutthe length of the metal cylinder along its longitudinal axis, betweenopposite ends of the fusible plug, and where the conduit is filled witha particular material (also referred to as a “fusible alloy”) that isconfigured to melt at a melting temperature that is less than themelting temperature of the body material of the body cylinder, such thatthe particular body material may partially or entirely melt when atemperature at least one end of the body cylinder reaches the meltingtemperature, such that the particular fusible alloy material may atleast partially flow out of the conduit to expose the conduit throughthe body cylinder and between the opposite ends of the fusible plug.Fusible plugs as described herein may include any well-known fusibleplugs, including, without limitation, fusible plugs having a bodymaterial that includes brass, bronze, steel, and/or gun metal, fusibleplugs having a fusible alloy that includes tin, any combination thereof,or the like.

The passive containment cooling system may include multiple coolantchannels that are coupled to the coolant reservoir via separate,respective coolant supply conduits and coolant return conduits, and thepassive containment cooling system may include one or more separatefirst check valve assemblies extending into separate, respective coolantconduits or other pathways to the coolant reservoir. Additionally,multiple check valve assemblies may extend into a given coolant channelor other pathway to the coolant reservoir, at a same or differentheights or depths within the given coolant channel or other pathway tothe coolant reservoir, and one or multiple fusible plugs may extend intoa given coolant channel or other pathway to the coolant reservoir, and asame or different heights or depths within the given coolant channel orother pathway to the coolant reservoir.

FIG. 1 is a cross-sectional schematic side view of a nuclear plant thatincludes a passive containment cooling system that further includes acontainment venting system and a containment flooding system, accordingto some example embodiments. FIGS. 2A-2C are expanded views of region Aof FIG. 1 , according to some example embodiments. FIG. 3 is an expandedview of region A of FIG. 1 , according to some example embodiments. FIG.5 is an expanded view of region B of FIG. 1 , according to some exampleembodiments. FIG. 6 is a perspective view of a passive containmentcooling system that includes one or more coolant channels integratedinto the containment structure, according to some example embodiments.

Referring to FIG. 1 , the nuclear plant 1 includes a reactor buildingstructure 110 on a foundation 2 (which may be the ground, bedrock, astructural foundation, any combination thereof, or the like), and anuclear reactor 100 within the reactor building structure 110. Thenuclear reactor 100 is within a containment environment 192 that issurrounded by, and is at least partially defined by, a containmentstructure 140 that provides pressure retention of the containmentenvironment 192. An inner surface 140 i of the containment structure 140may at least partially define the containment environment 192. Thecontainment structure 140 may be a solid structure, comprised of one ormore pieces of material coupled together, and may include metal and/orconcrete material pieces. In some example embodiments, the containmentstructure 140 may be a steel-concrete composite (SC) structure, as theterm is well-known.

As shown in FIG. 1 , the nuclear plant 1 includes a passive containmentcooling system 200 that is configured to provide passive cooling andcontainment of the containment environment 192, and the containmentfluid 197 included therein, and of the nuclear reactor 100 includedtherein. The passive containment cooling system 200 includes a coolantreservoir 120, one or more coolant supply conduits 150, one or morecoolant channels 160 coupled to the containment structure 140, and oneor more coolant return conduits 170. As shown, the passive containmentcooling system 200 may include one or more check valve assemblies 180,but example embodiments are not limited thereto. The passive containmentcooling system 200 is configured to provide passive cooling of thecontainment environment 192 based on inducing and/or maintaining a flowor circulation of coolant fluid 122, 124, 125, 126 between the coolantreservoir 120 and the one or more coolant channels 160 to absorb heat102 rejected by the nuclear reactor 100 and remove the heat to thecoolant reservoir 120.

As shown in FIG. 1 , the passive containment cooling system 200 mayinclude multiple coolant channels 160 that are coupled to separateportions of the containment structure 140 and which are coupled to thecoolant reservoir 120 via separate, respective coolant supply conduits150 and separate, respective coolant return conduits 170, and whereseparate, respective check valve assemblies 180 extend into separate,respective coolant channels 160, or one or more other pathways to thecoolant reservoir 120, and, if needed, through the containment structure140 thickness 141 to the separate, respective coolant channels 160 orthe one or more other pathways to the coolant reservoir 120, and whereseparate, respective fusible plugs 190 extend into the separate,respective coolant channels 160 or one or more other pathways to thecoolant reservoir 120 and, if needed, through the containment structure140 thickness 141 to the separate, respective coolant channels 160 orthe one or more other pathways to the coolant reservoir 120. Thefollowing description is directed to a single coolant channel 160 andthe respective conduits 150, 170 and check valve assemblies 180, 380 andfusible plugs 190 extended thereinto, but it will be understood thatsaid description may apply to all of the coolant channels 160, conduits150, 170, check valve assemblies 180, 380 (second check valve assembly380 is shown in FIG. 3 ), and fusible plugs 190 of the passivecontainment cooling system 200.

As shown in FIG. 1 , the coolant reservoir 120 is located verticallyabove the nuclear reactor 100, such that a top of the nuclear reactor100 is located at a vertical height H1, and the bottom 120 b of thecoolant reservoir 120 is at a vertical height H2, where H2 is greaterthan H1. Accordingly, any fluid held in the coolant reservoir 120 mayflow downwards (e.g., flow downwards or “fall” in the direction ofgravitational acceleration “g”) from the coolant reservoir 120 to aheight of any portion of the nuclear reactor 100.

All heights H1 to H6 as described herein will be understood to beheights measured from a single, fixed reference height H0. Asillustrated in FIG. 1 , the heights H1 to H6 are shown to be heightsfrom a top surface of the foundation 2 at a height H0, such that the topsurface of the foundation 2 provides the reference height H0 via whichthe heights H1 to H6 of other elements in the nuclear plant 1 may bedescribed and compared. But, it will be understood that, in some exampleembodiments, the top surface of the foundation 2 may have a variableheight, and the heights H1 to H6 described herein may be understood tobe heights from a single, constant reference height H0 that may bedifferent from the height of the top surface of the foundation 2 (e.g.,a height of global mean sea level (MSL), as the term is well-known).

As shown in FIG. 1 , the coolant reservoir 120 is configured to hold(e.g., be filled with) a coolant fluid 122, such that the top surface122 t of the coolant fluid 122 in the coolant reservoir 120 is at adepth D122 above the height (H2) of the bottom 120 b of the coolantreservoir 120. Accordingly, it will be understood that the hydrostaticpressure of the coolant fluid 122 at the bottom 120 b of the coolantreservoir 120 is equal to a pressure head of coolant fluid 122 having aheight equal to depth D122. As shown in FIG. 1 , and as describedfurther below, the coolant reservoir 120 may be considered to have anupper region 121 a and a lower region 121 b that is below the upperregion 121 a (e.g., proximate to the bottom 120 b and distal to the topsurface 122 t in relation to the upper region 121 a). Additionally, thecoolant fluid 122 held in the coolant reservoir 120 may include coolantfluid 123 a, that is defined as the portion of the coolant fluid 122that is within the upper region 121 a, and lower coolant fluid 123 b,that is defined as the portion of the coolant fluid 122 that is withinthe lower region 121 b.

As shown in FIG. 1 , a coolant supply conduit 150 is coupled to thecoolant reservoir 120, such that an inlet 152 of the coolant supplyconduit 150 is open to the lower region 121 b of the coolant reservoir120 (e.g., opens directly into the lower region 121 b of the coolantreservoir 120) and the coolant supply conduit 150 extends downwards(e.g., in the direction of gravitational acceleration “g”) from theinlet 152, downwards from the bottom 120 b of the coolant reservoir 120,to an outlet 154. As shown, the inlet 152 may be at a vertical heightH3, and the outlet 154 may be at a vertical height H4, where H3 isgreater than H4, H4 is less than H1, and where H3 is equal to or greaterthan H2. Accordingly, it will be understood that, based on extendingdownwards from the bottom 120 b of the coolant reservoir 120, thecoolant supply conduit 150 may be configured to direct at least some ofthe coolant fluid 122 in the coolant reservoir 120 (e.g., coolant fluid123 b in the lower region 121 b) to flow, as coolant fluid 124,downwards (e.g., at least partially in the direction of gravitationalacceleration “g”) from the coolant reservoir 120 and into the coolantsupply conduit 150 via inlet 152, and to flow at least partiallydownwards (e.g., “fall”) through the coolant supply conduit 150 to theoutlet 154, according to gravity (e.g., gravitational acceleration).Accordingly, it will be understood that a flow of coolant fluid 124through the coolant supply conduit 150 may be induced and/or maintainedaccording to gravity, and thus may be induced and/or maintained withoutoperation of any active flow generators (e.g., pumps) and without (e.g.,independently of) operator or control system intervention and thus theflow may be considered to be “passive.”

As shown in FIG. 1 , the inlet 152 of the coolant supply conduit 150 maybe elevated above the bottom 120 b of the coolant reservoir 120 by aspacing height H152. In FIG. 1 , H152 is shown to be a positive value,such that the height H3 of the inlet 152 is greater than the height H2of the bottom 120 b of the coolant reservoir 120. But, it will beunderstood that, in some example embodiments, the inlet 152 may be atthe same height as the bottom 120 b of the coolant reservoir 120 (e.g.,H2 may equal H3), such that height H152 may be a null value.Additionally, while FIG. 1 illustrates the bottom 120 b of the coolantreservoir 120 as being a flat, horizontal surface (e.g., beingperpendicular to the direction of gravitational acceleration “g”), itwill be understood that example embodiments are not limited thereto, andin some example embodiments the height H3 of the bottom 120 b may beunderstood to be a lowest height of the bottom 120 b of the coolantreservoir 120. For example, in some example embodiments, the bottom 120b may be angled (e.g., have a truncated conical shape) where the inlet152 is at the height of the lowest portion of the bottom 120 b (e.g.,H3=H2), so that coolant fluid 123 b in the lowest portion of the coolantreservoir 120 may be drawn downwards, into the inlet 152 according togravity.

Still referring to FIG. 1 , the nuclear plant 1 includes one or morecoolant channels 160 that are coupled to the containment structure 140,such that each coolant channel 160 extends vertically along thecontainment structure 140, from a coolant channel inlet 162 at a bottomof the coolant channel 160 to a coolant channel outlet 164 at a top ofthe coolant channel 160.

In FIG. 1 , the coolant channels 160 are illustrated as conduits (e.g.,pipes) coupled to the outer surface 140 o of the containment structure140 (which may be implemented via any well-known methods of joiningconduits to separate structures. It will be understood that the coolantchannels 160, in some example embodiments, may be coupled to the innersurface 140 i of the containment structure 140 instead of the outersurface 140 o, for example to satisfy one or more physical constraints).But, it will be understood that example embodiments of coolant channels160 are not limited thereto. For example, turning to FIG. 6 , in someexample embodiments, a containment structure 140 may include aconcentric arrangement of an inner cylindrical shell 642 and an outercylindrical shell 644, where the inner surface 642 i of the innercylindrical shell at least partially defines the containment environment192, and where the outer surface 642 o of the inner cylindrical shell642 and the inner surface 644 i of the outer cylindrical shellcollectively define an annular gap space 648 in which one or morecoolant channels 160 may be defined, e.g., by surfaces 642 o and 644 ialone or in combination with additional structural surfaces. Forexample, in FIG. 6 , one or more column structures 646 extend verticallythrough the annular gap space 648, and further extend completely betweensurfaces 642 o and 644 i, to azimuthally partition the annular gap space648 into multiple, isolated coolant channels 160, where a given coolantsupply conduit 150 and coolant return conduit 170 may be coupled to aparticular coolant channel 160. The coolant channels 160 shown in FIG. 6, being defined by the structures 642, 644, 646 that at least partiallycomprise the containment structure 140, extend through the interior ofthe containment structure 140 and may be understood to be integratedinto the containment structure 140.

As shown in FIG. 1 , the inlet 162 of a coolant channel 160 may becoupled to an outlet 154 of the coolant supply conduit 150, such thatthe inlet 162 is at a same height as the height of the outlet 154 of thecoolant supply conduit 150: height H4. As further shown, the height H5of the outlet 164 of the coolant channel 160 at the top of the coolantchannel 160 may be less than the height H2 of the bottom 120 b of thecoolant reservoir 120, but example embodiments are not limited theretoand in some example embodiments the coolant channel 160 may extendvertically up and above the height of the bottom 120 b of the coolantreservoir 120, such that H5 may be greater than H2. In some exampleembodiments, the coolant return conduit 170 as described herein may beincorporated into an upper portion of the coolant channel 160 thatextends above the height H1 of the nuclear reactor 100 to the height H6of the outlet 174.

As shown in FIG. 1 , coolant fluid 124 that is directed to fall throughthe coolant supply conduit 150 to the outlet 154 according to gravitymay be directed into the bottom of a coolant channel 160, at height H4via the inlet 162 that is coupled to the outlet 154. As shown, thecoolant channel 160 is coupled to the containment structure 140 and thusis configured to receive heat 102 rejected by the nuclear reactor 100and through the containment environment 192 via at least a portion ofthe containment structure 140. The coolant fluid 124 that is in thecoolant channel 160 may absorb at least some of the heat 102 and thusmay become a heated coolant fluid 125. The heated coolant fluid 125within the coolant channel 160 may have a change in buoyancy (e.g.,change in density) based on absorbing said heat 102, such that thebuoyancy of the heated coolant fluid 125 is increased (and density isreduced) in relation to the colder coolant fluid 124 that is beingdirected into the bottom of the coolant channel 160 via the coolantsupply conduit 150.

As a result, the heated coolant fluid 125 may rise (e.g., flow upwards,at least partially in a direction that is opposite the direction ofgravitational acceleration “g”), from the bottom of the coolant channel160 at height H4 to the top of the coolant channel 160 at height H5,based on having said increased buoyancy (e.g., reduced density), whilethe heated coolant fluid 125 is displaced at the bottom of the coolantchannel 160 by the colder (and thus less buoyant and denser),newly-supplied coolant fluid 124 via the coolant supply conduit 150. Itwill be understood that the upwards flow (e.g., rising) of the heatedcoolant fluid 125 in the coolant channel 160 may be considered to be a“passive” driving of coolant fluid flow, as the flow, being induced byabsorbing heat rejected from the nuclear reactor 100, is not beingdriven by an active flow generator (e.g., a pump), and is not beingdriven due to operator intervention to specifically control coolantfluid flow. It will be considered that any operator intervention and/ordevice operation in the nuclear plant 1 that adjust the heat rejection102 by the nuclear reactor 100, which may indirectly affect coolantfluid 125 flow due to heat 102 absorption, is not considered herein tobe operator intervention and/or device operation in the nuclear plant 1to control coolant fluid flow.

Still referring to FIG. 1 , a coolant return conduit 170 is coupled, atan inlet 172, to the outlet 164 of a coolant channel 160 at the top ofthe coolant channel 160 (e.g., at height H5) and extends upwards to anoutlet 174 that is at a greater height H6. As further shown in FIG. 1 ,the outlet 174 is open to the upper region 121 a of the coolantreservoir 120. FIG. 1 illustrates the coolant return conduit 170 asextending upwards through the bottom 120 b of the coolant reservoir 120to a height H174 above the bottom 120 b, such that the outlet 174 isupwards facing, similarly to the inlet 152 of the coolant supply conduit150. But, it will be understood that example embodiments are not limitedthereto. For example, the coolant return conduit 170 may extend upwardsto height H6 and may turn and extend through a sidewall 120 s of thecoolant reservoir 120 so that the outlet 174 faces sideways (e.g.,perpendicular to the direction of gravitational acceleration “g”).

As shown, heated coolant fluid 125 that rises to the top of the coolantchannel 160, at height H5, may be directed through outlet 164, and thusthrough the coupled inlet 172, such that the coolant return conduit 170may be configured to direct a flow of the heated coolant fluid 125 torise out of the top of the coolant channel 160 via the coolant channeloutlet 164, as coolant fluid 126, and into the coolant reservoir 120 viaconduit 170 and outlet 174, according to the increased buoyancy of thehotter coolant fluid 126 (due to having absorbed heat 102 as heatedcoolant fluid 125) at the top of the coolant channel 160 (e.g., atheight H5) over a buoyancy of the colder coolant fluid 124 at the bottomof the coolant channel 160. In some example embodiments, the coolantsupply conduit 150 and/or coolant return conduit 170 may be partially orcompletely insulated so as to mitigate heat loss by the coolant fluid126 which might affect the upwards, buoyancy-driven flow of coolantfluid 126 and/or the downwards, gravity and/or density-driven flow ofcoolant fluid 124.

Referring back to the coolant reservoir 120, the coolant reservoir 120may be considered to be vertically divided into an upper region 121 aand a lower region 121 b, where the interface between the upper andlower regions 121 a and 121 b may be any height between the height H174of the coolant return conduit outlet 174 from the bottom 120 b of thecoolant reservoir 120 (at height H2) and the height H152 of the coolantsupply conduit inlet 152 from the bottom 120 b of the coolant reservoir120 (at height H2). In some example embodiments, the height H121 of theinterface between the upper and lower regions 121 a and 121 b is aheight H121 that is equally vertically distant from (e.g., halfwaybetween) height H152 and height H174. In some example embodiments, theheight H121 of the interface between the upper and lower regions 121 aand 121 b is the height H174 of the outlet 174 in the coolant reservoir120, such that all portions of the coolant reservoir between height H2and H6 are the lower region 121 b, and all portions of the coolantreservoir 120 at or above height H6 are the upper region 121 a.

As shown in FIG. 1 , the coolant supply conduit inlet 152 is open to thelower region 121 b of the coolant reservoir 120, and the coolant returnconduit outlet 174 is open to the upper region 121 a of the coolantreservoir 120. It will be understood that, due to warmer coolant fluid122 having increased buoyancy over colder coolant fluid 122, the coolantfluid 123 a in the upper region 121 a of the coolant reservoir 120 maybe warmer, and have increased buoyancy, over the coolant fluid 123 b inthe lower region 121 b of the coolant reservoir 120. It will beunderstood that warmer coolant fluid 126 may have an increased buoyancy(e.g., reduced density) over colder coolant fluid 122 in the coolantreservoir 120, including the coolant fluid 123 b in the lower region 121b of the coolant reservoir 120. Accordingly, warmer coolant fluid 126may occupy the upper region 121 a, becoming part of the warmer, morebuoyant coolant fluid 123 a, while colder coolant fluid 122 may occupythe lower region 121 b as coolant fluid 123 b.

Thus, as shown in FIG. 1 , colder coolant fluid 122 occupying the lowerregion 121 b of the coolant reservoir 120 (e.g., coolant fluid 123 b)may be drawn into the coolant supply conduit 150, according to gravityand having less buoyancy (e.g., greater density) than the warmer coolantfluid 123 a, via the inlet 152 that is open to (e.g., located within)the lower region 121 b. Accordingly, the passive containment coolingsystem may supply colder, higher-density coolant fluid 123 b to thecoolant channel 160 as coolant fluid 124, while warmer coolant fluid 126may be caused to rise above the colder coolant fluid 123 b, as coolantfluid 123 a, and thus be isolated from being inadvertently drawn intothe coolant supply conduit 150 via inlet 152, based on the increasedbuoyancy (e.g., reduced density) of the coolant fluid 126 over thecolder coolant fluid 123 b in the lower region 121 b. Furthermore, asshown in FIG. 1 , the coolant return conduit outlet 174, beingvertically higher in the coolant reservoir 120 than the inlet 152 by avertical distance of dH152, is open to (e.g., located within) the upperregion 121 a, such that warmer, lower-density coolant fluid 126 issupplied directly into the upper region 121 a to mix with, and becomepart of, coolant fluid 123 a without mixing with the colder coolantfluid 123 b in the lower region 121 b. The coolant fluid 126 may remainin the upper region 121 a as part of coolant fluid 123 a, and thusremain isolated from inlet 152, due to having the increased buoyancy dueto being warmer than coolant fluid 123 b. Over time, at least some ofthe coolant fluid 123 a may cool and may circulate into the lower region121 b to become coolant fluid 123 b and thus to eventually be drawn backinto the coolant supply conduit 150.

As further shown in FIG. 1 , the nuclear plant 1 may include a heatremoval system (e.g., heat exchanger 128, which may be any well-knownheat exchanger device) that is configured to remove the heat 102introduced into coolant reservoir 120 by the coolant fluid 126, tothereby mitigate or prevent the risk of the coolant fluid 122 warmingup, and thus potentially degrading the ability of the passivecontainment cooling system 200 to remove heat 102 from the containmentenvironment 192. However, it will be understood that the coolantreservoir 120 may, at least temporarily, serve as a heat sink that mayabsorb and retain the heat 102 that is removed into the coolantreservoir via the coolant fluid 126, for at least a period of time,without the heat 102 being removed from the coolant reservoir 120 viaoperation of any heat exchanger 128. Accordingly, the passivecontainment cooling system 200 may enable a passively-driven (e.g.,driven by gravity via coolant supply conduit 150 and via absorbed heatvia coolant channel 160 and coolant return conduit 170) circulation ofcoolant fluid 122, 124, 125, 126 between the coolant reservoir 120 andthe coolant channel 160 to remove heat 102 from the containmentenvironment 192. It will be understood that absorbing heat 102 that isrejected by the nuclear reactor 100 via the containment environment 192and at least a portion of the containment structure 140, as performed bythe coolant fluid 125, amounts to removing heat from the containmentenvironment 192.

It will be understood that the flow rate of the coolant fluid 124, 125,126 may be at least partially driven by the rate of heat rejection 102by the nuclear reactor 100. Accordingly, the rate of heat removal fromthe containment environment 192 and into the coolant reservoir 120 bythe coolant fluid may be proportional to the coolant flow rate throughconduits 150, 170, and coolant channel 160, and such flow rate may bedriven by and proportional to the rate of heat rejection 102 by thenuclear reactor 100. Such variation of flow and heat removal by thepassive containment cooling system 200 may be partially or entirelydriven by the rate of heat rejection 102 by the nuclear reactor 100 andmay be performed without (e.g., independently of) any operatorintervention in the nuclear plant 1, even without any such interventionwith regard to operation of the nuclear reactor 100. Accordingly, thepassive containment cooling system 200 may enable regulation of thetemperature and/or pressure of the containment environment 192, at leasttemporarily, without (e.g., independently of) any operator or controlsystem intervention.

Still referring to FIG. 1 , and referring further to FIGS. 2A-2C, thepassive containment cooling system 200 may include a first check valveassembly 180 at a position that is a vertical depth DB180 below a bottom120 b of the coolant reservoir 120 and thus at a vertical depth DT180below a top surface 122 t of coolant fluid 122 in the coolant reservoir120, where the first check valve assembly 180 is in fluid communicationwith both the coolant reservoir 120 and with the containment environment192. As shown in FIG. 1 , the first check valve assembly 180 may extendthrough the thickness 141 of the containment structure 140 (e.g.,between surfaces 140 i and 140 o) and into the coolant channel 160, soas to be in fluid communication with the coolant reservoir 120 via thecoolant channel 160, but example embodiments are not limited thereto andthe outlet 180 o of the first check valve assembly 180 may be open toanother, separate conduit (also referred to interchangeably herein as a“pathway”) other than any coolant channel 160 at vertical depthDB180/DT180, where the other, separate conduit is in fluid communicationwith the coolant reservoir 120 and thus establishes fluid communicationbetween the first check valve assembly 180 and the coolant reservoir120. It will be understood that the vertical depth DT180 is equal to asum of the vertical depth DB180 and the coolant reservoir depth D122 ofcoolant fluid 122, from the bottom 120 b to the top surface 122 t, inthe coolant reservoir 120. The first check valve assembly 180 mayinclude one or more check valves 182 coupled between a first check valveassembly inlet 180 i, via an inlet conduit 181 i, and a check valveassembly outlet 180 o, via an outlet conduit 181 o. As shown, the firstcheck valve assembly inlet 180 i is open to the containment environment192, and the first check valve assembly outlet 180 o is in fluidcommunication with the coolant reservoir 120 at vertical depthDB180/DT180 (e.g., is open to the coolant channel 160 or any otherconduit to the coolant reservoir 120 at the vertical depth DB180/DT180).

In some example embodiments, the one or more check valves 182 areconfigured to actuate to open (e.g., actuate from a closed state to anopen state), thereby establishing a continuous flow conduit 187 (alsoreferred to herein as a fluid conduit) between the inlet 180 i and theoutlet 180 o and thus enabling a one-way flow 198 of some or all fluidslocated in the containment environment 192, such fluids being referredto herein as a containment fluid 197, to the coolant reservoir 120 inresponse to a magnitude of the pressure at the inlets 182 i of the oneor more check valves 182 being equal to or greater than a firstthreshold magnitude (e.g., PX1). Such configuration may be based on theone or more check valves 182 being structurally configured (e.g., basedon including a spring-loaded actuator) to open in response to thepressure at the inlet 182 i of the one or more check valves 182 beingequal to or greater than the first threshold magnitude PX1) As shown inFIG. 1 and FIGS. 2A-2C, the one or more check valves 182 may have aninlet 182 i that is coupled, via an inlet conduit 181 i, to the inlet180 i that is open to the containment environment 192, such that thepressure at the inlet 182 i of the one or more check valves 182 may bethe same as (e.g., equal to) the pressure P192-1 of the containmentenvironment at the inlet 180 i of the first check valve assembly 180.Thus, the one or more check valves may be configured to open in responseto the pressure P192-1 reaching (e.g., being equal to or greater than)the first threshold magnitude PX1.

The first threshold magnitude PX1 may at least partially correspond to ahydrostatic pressure P180 of the coolant fluid 125 in the coolantchannel 160 or other similar pathway to the coolant reservoir 120 at thefirst check valve assembly outlet 180 o at the vertical depthDB180/DT180. It will be understood that the hydrostatic pressure P180may be equal to a pressure head of the coolant fluid 122, 124, 125,and/or 126 having a height equal to the vertical depth DT180. In someexample embodiments, the coolant reservoir 120 may be configured to befilled with coolant fluid 122 to a reservoir depth D122 such that thetop surface 122 t of the coolant fluid 122 in the coolant reservoir 120is at a particular depth D122 above the bottom 120 b of the coolantreservoir 120 throughout operation of the nuclear plant 1.

In some example embodiments, the first reservoir depth D122 may varybased on the variation in amount of coolant fluid 122 in the coolantreservoir 120. In some example embodiments, a reference hydrostaticpressure P180 may be a hydrostatic pressure P180 that results from thecoolant reservoir 120 being filled to a particular, reference depthD122, such that the reference hydrostatic pressure P180 may be equal toa pressure head of the coolant fluid 122, 124, 125, and/or 126 having aheight equal to the vertical depth DT180 when the coolant reservoir 120is filled with coolant fluid 122 to the particular reference depth D122.The one or more check valves 182 may be configured to actuate to theopen state in response to the magnitude of the pressure at the one ormore inlets 182 i (and thus, for at least one of the check valves 182,the pressure P192-1 of the containment environment at the inlet 180 i)reaching (e.g., being equal to or greater than) a first thresholdmagnitude PX1 that is at least greater than the reference hydrostaticpressure P180, such that, when the coolant reservoir 120 is filled tothe particular reference depth D122, a pressure gradient is presentacross the one or more check valves 1182 when the magnitude of thepressure P192-1 reaches the first threshold magnitude PX1. It will beunderstood that, due to variation at any given time in the depth D122 towhich the coolant reservoir 120 may be filled with coolant fluid 122,the first threshold magnitude PX1 may be set to be a magnitude that isat least a particular margin (e.g., 5% greater, 10% greater, 20%greater, a particular additional amount of pressure, any combinationthereof, or the like) greater than the reference hydrostatic pressureP180 (e.g., the hydrostatic pressure P180 at the outlet 180 o when thecoolant reservoir 120 is filled to the particular reference depth D122),to improve the likelihood that the actual hydrostatic pressure P180 willbe less than the first threshold magnitude PX1 at the inlets 182 i ofthe one or more check valves 182 when the magnitude of the pressureP192-1 reaches the first threshold pressure PX1, thereby ensuring that apressure gradient is present across the first check valve assembly 180from the inlet 180 i to the outlet 180 o. It will be understood that,because the inlet 180 i of the first check valve assembly 180 is at thesame vertical depths DB180/DT180 as the rest of the first check valveassembly 180, the pressure P192-1 of the containment environment 192 atthe inlet 180 i may be understood to be a pressure P192-1 of thecontainment environment 192 at the vertical depth DB180/DT180.

The one or more check valves 182 may be configured to selectively (e.g.,reversibly) actuate based on whether a pressure at the inlet 182 i ofthe one or more check valves 182 is equal to or greater than the firstthreshold magnitude PX1. Accordingly, the first check valve assembly 180may be configured to selectively open a flow conduit 187 to selectivelyenable one-way flow 198 of a containment fluid 197, from the containmentenvironment 192 to the coolant reservoir 120 via the first check valveassembly 180 and one or more coolant channels 160, or other pathway tothe coolant reservoir 120, to which the outlet 180 o is open, based onthe one or more check valves 182 actuating to open (e.g., opening) inresponse to a pressure P192-1 of the containment environment 192 at thefirst check valve assembly inlet 180 i at the vertical depth DB180/DT180being equal to or greater than the first threshold magnitude PX1. Theone-way direction of the one-way flow 198 may be ensured, therebypreventing backflow through the first check valve assembly 180 from thecoolant channel 160 or other pathway into the containment environment192, based on the first check valve assembly 180 defining the flowconduit 187 from inlet 180 i to outlet 180 o to extend through the oneor more check valves 182, where the one or more check valves 182 areconfigured to enable one-way flow in the direction from inlet 180 i tooutlet 180 o, and the one or more check valves 182 each being configuredto open in response to the pressure at the inlet 182 i of the checkvalve 182 at least reaching the first threshold pressure PX1 that isgreater than a reference hydrostatic pressure P180 of the coolant fluid125 at the outlet 180 o at the vertical depth DB180/DT180 (e.g., ahydrostatic pressure of coolant fluid 125 that is equal to a pressurehead of the coolant fluid at a height equal to depth DT180).

In some example embodiments, the one or more check valves 182 maysubsequently close, once pressure P192-1 drops below the first thresholdmagnitude PX1. Accordingly, the one-way flow of containment fluid 197may be selectively enabled and inhibited to regulate the pressure withinthe containment environment.

The selective enabling of one-way flow 198 of containment fluid 197 maybe referred to herein as “venting” of the containment fluid 197, forexample to regulate the pressure (e.g., P192-1) in the containmentenvironment 192 and thus to mitigate or prevent the risk of overpressureof the containment structure 140.

Operation (e.g., actuation) of the one or more check valves 182 of thefirst check valve assembly 180 may occur without (e.g., independentlyof) any operator intervention. Accordingly, the pressure relief, or“venting” functionality provided by the first check valve assembly 180may be understood to be “passive.”

In some example embodiments, where the outlet 180 o of the first checkvalve assembly 180 is open to a coolant channel 160 at vertical depthDB180/DT180, the containment fluid 197, which may include radioactivematerial, solids, gasses, liquids, any combination thereof, or the like,may be entrained in the rising flow of the heated coolant fluid 125through the coolant channel 160 and may be thus drawn into the coolantreservoir 120 with the coolant fluid 126. Similarly, where the outlet180 o is open to another pathway to the coolant reservoir 120, thecontainment fluid 197 may pass from the first check valve assembly 180to the reservoir 120 via the other pathway. The coolant fluid 125, 126,122 may quench some gases in the containment fluid 197 (e.g., steam) tothereby reduce the pressure in the coolant reservoir 120, and otherparts of the containment fluid 197 may be retained in the coolantreservoir 120, at least temporarily, to reduce or prevent venting orescape of containment fluid 197 to the ambient environment external tothe nuclear plant 1. Accordingly, the first check valve assembly 180 mayenable improved passive containment of containment fluid while enablingpassive regulation of pressure in the containment environment 192.

While FIG. 1 illustrates one or more first check valve assemblies 180extending into one or more coolant channels 160, it will be understoodthat example embodiments are not limited thereto. For example, one ormore first check valve assemblies 180 of the passive containment coolingsystem 200 may, instead of extending into a coolant channel 160, berouted to the coolant reservoir 120 via one or more other, separateconduits, which may also be referred to as pathways, parallel pathways,or the like, into which the one or more first check valve assemblies 180may extend. For example, a first check valve assembly 180 may extend,from the containment environment 192, into a separate conduit, alsoreferred to as a separate pathway or parallel pathway (not shown in FIG.1 ) that may extend to the coolant reservoir 120 independently of theone or more coolant channels 160. Accordingly, in some exampleembodiments, one or more first check valve assemblies 180 may beconfigured to enable “venting” of one or more one-way flows 198 ofcontainment fluid 197 to the coolant reservoir 120 independently of(e.g., in parallel with) the one or more coolant channels 160, therebyenabling the coolant reservoir 120 to retain at least some of thematerial of the coolant fluid 197, independently of the one or morecoolant channel 160.

Referring now, generally, to FIGS. 2A-2C, the first check valve assembly180 may include one or more various configurations of one or more checkvalves 182 shown therein, although example embodiments are not limitedthereto.

As shown in FIG. 2A, the first check valve assembly 180 may include asingle check valve 182 having an inlet 182 i that is that is coupled tothe first check valve assembly inlet 180 i via inlet conduit 181 i, andthus inlet 182 i is open to inlet 180 i, and an outlet 182 o that iscoupled to the first check valve assembly outlet 180 o via outletconduit 181 o, and thus outlet 182 o is open to outlet 180 o. Thus, insome example embodiments, a pressure P192-1 at the inlet 180 i may bethe pressure at the inlet 182 i of the single check valve 182 of thefirst check valve assembly 180, and the check valve 182 may beconfigured to actuate from the closed state to the open state inresponse to the pressure at the inlet 182 i reaching a first thresholdmagnitude PX1. Thus, the check valve 182 may actuate to open in responseto the pressure P192-1 at the inlet 180 i reaching the first thresholdmagnitude PX1, to thereby cause the first check valve assembly 180 toselectively establish an open flow conduit 187 between inlet 180 i andoutlet 180 o via the check valve 182 and conduits 181 i and 181 o, andthus selectively enable the one-way flow 198 of containment fluid 197,based on the pressure P192-1 reaching the first threshold magnitude PX1.

Referring to FIG. 2B, in some example embodiments, the one or more checkvalves 182 may include a series connection of a plurality of checkvalves 182-1 to 182-i (e.g., a series connection of “i” check valves,where “i” is a positive integer having a value equal to or greater than2) between the first check valve assembly inlet 180 i and the firstcheck valve assembly outlet 180 o. As shown in FIG. 2B, the outlets 182o of check valves 182-1 to 182-(i-1) may be coupled to the adjacentinlets 182 i of adjacent check valves in the series connection viaintermediate conduits 183-1 to 183-(i-1). Each check valve 182 of theplurality of check valves 182-1 to 182-i may be configured to actuate toopen in response to a pressure at an inlet 182 i of the each check valve182 being equal to or greater than the first threshold magnitude PX1.Similarly to FIG. 2A, the inlet 182 i of the first check valve 182-1 inthe series connection may be coupled to, and open to, the inlet 180 ivia inlet conduit 181 i, and the outlet 182 o of the last check valve181-i in the series connection may be coupled to, and open to, theoutlet 180 o via outlet conduit 181 o. Accordingly, when the pressureP192-1 at inlet 180 i, reaches the first threshold magnitude PX1, thefirst check valve 182-1 may actuate to open, as the pressure at theinlet 182 i of the first check valve 182-1 may be the same as thepressure at inlet 180 i, and then the next check valves 182-2 to 182-iin the series connection may actuate to open in succession in responseto each preceding check valves 182 in the series connection opening andestablishing fluid communication between the inlet 182 i of thesucceeding check valve 182 in the series connection with inlet 180 i,until all check valves 182-1 to 182-i are opened and the flow conduit187 between inlet 180 i and outlet 180 o via check valves 182-1 to 182-iis established. Thus, the first check valve assembly 180 may selectivelyenable the one-way flow 198 of containment fluid 197 based on all checkvalves 182 of the series connection of the plurality of check valvesopening 182-1 to 182-i selectively actuating to open. Additionally, theone-way flow 198 may be inhibited in response to any of the check valves182-1 to 182-i being closed. Thus, if the pressure P192-1 subsequentlydrops below the first threshold magnitude PX1 after initially reachingthe first threshold magnitude PX1, the series connection of check valves182-1 to 182-i may reduce the risk that the flow conduit 187 betweeninlet 180 i and outlet 180 o might remain open, as the closure of anyone of the check valves 182-1 to 182-i would close the flow conduit 187and inhibit the one-way flow 198. Thus, the series connection shown inFIG. 2B may reduce the risk of inadvertent backflow from the coolantchannel 160 or other pathway to which the outlet 180 o is open and intothe containment environment 192 via the first check valve assembly 180,thereby improving reliability of the passive containment cooling system200.

Referring to FIG. 2C, in some example embodiments, the one or more checkvalves 182 may include a parallel connection of a plurality of checkvalves 182-1,1 to 182-i,j between inlet 180 i and one or more outlets180 o-1 to 180 o-j (e.g., a parallel connection of “j” sets of seriesconnections of “i” check valves with at least inlet 180 i, where “j” isa positive integer that is equal to or greater than 1 and “i” is apositive integer that is equal to or greater than 1). As shown in FIG.2C, the inlets 182 i of check valves 182-1,1 to 182-1,j may be coupledin parallel to the inlet 180 i via inlet conduit 181 i and separate,respective inlet branch conduits 281-1 to 281-j. As further shown, eachseparate branch (1 to j) of one or more (e.g., “i”) check valves 182 maybe coupled in series between inlet 180 i and a separate outlet 180 o-1to 180 o-j, similarly to the series connection of check valves 182-1 to182-i as described with reference to FIG. 2B. But, example embodimentsare not limited thereto, and in some example embodiments, two or morebranches 1 to j of check valves 182 may be coupled in parallel between asingle inlet 180 i and a single outlet 180 o, via one or more branchinlet conduits 281-1 to 281-j and one or more branch outlet conduits282-1 to 282-j.

Each check valve 182 of the plurality of check valves 182-1,1 to 182-i,jmay be configured to open in response to a pressure at an inlet 182 i ofthe each check valve 182 being equal to or greater than the firstthreshold magnitude PX1. Accordingly, when the pressure P192-1 at inlet180 i reaches the first threshold magnitude PX1, each of the checkvalves 182-1,1 to 182-i,j may open, as the pressure at the inlet 182 iof each check valve 182-1,1 to 182-1,j may be the same as the pressureat inlet 180 i and each series connection of one to i check valves ineach parallel branch of check valves 182 may actuate in succession asdescribed above with reference to FIG. 2B, thereby establishingmultiple, parallel fluid conduits 187-1 to 187-j between inlet 180 i andone or more outlets 180 o-1 to 180-j. Thus, the first check valveassembly 180 may selectively enable the one-way flow 198 of containmentfluid 197 based on any set of one or more check valves of the parallelconnection of sets of one or more check valves 182-1,1 to 182-i,jactuating to open. Where “i” equals 1, such that the first check valveassembly 180 includes a parallel connection of check valves 182-1 to182-j, the first check valve assembly 180 may selectively enable theone-way flow 198 of containment fluid 197 based on any check valve 182of the parallel connection of check valves 182-1 to 182-j actuating toopen Accordingly, in some example embodiments, the one-way flow 198 maybe ensured, even if one or more of the check valves 182-1 to 182-j donot open, so long as at least one (e.g., any) of the check valves 182-1to 182-j open.

Still referring to FIGS. 2A-2C, in some example embodiments a checkvalve assembly includes a burst disc 186 coupled between the inlet 182 iof the one or more check valves 182 and the inlet 180 i of the firstcheck valve assembly 180. For example, as shown in FIGS. 2A-2C, theburst disc 186 may be coupled in series with the one or more checkvalves 182 of the first check valve assembly 180. The burst disc 186,also known as a pressure safety disc, rupture disk, bursting disc, burstdiaphragm, or the like, may be any well-known type of burst disc used toprovide a non-reclosing pressure relief flow control (e.g., pressurerelief) device. In some example embodiments, the burst disc 186 isconfigured to rupture in response to a pressure at the inlet side 186 iof the burst disc 186 reaching the first threshold pressure PX1, or anyother particular pressure threshold magnitude (e.g., a particular, or,alternatively, pre-determined “set point” threshold). Because the burstdisc 186 may be between the inlet 182 i of the first check valve 182 ofthe one or more check valves 182 as shown in FIGS. 2A to 2C, the inletside 186 i of the burst disc 186 is in open fluid communication with(e.g., open to) inlet 180 i, such that the pressure P192-1 at inlet 180i is also the pressure at the inlet side 186 i of the burst disc 186,and thus the burst disc 186 is configured to rupture if the pressureP192-1 reaches the first threshold magnitude PX1, or any otherparticular pressure threshold magnitude, to cause the pressure at theinlet 182 i of one or more check valves 182 to reach the pressure P192-1at inlet 180 i, and thus the one or more check valves 182 may actuate tothe open state to enable one-way flow 198 of the containment fluid 197therethrough in response to the pressure P192-1 reaching the firstthreshold magnitude PX1. The burst disc 186 may provide an additionallevel of reliability to the first check valve assembly 180 based onpreventing premature establishment of the flow conduit 187 through thefirst check valve assembly 180 if pressure P192-1 has not reached thefirst threshold magnitude PX1 at least once.

Referring back to FIG. 1 , while the passive containment cooling system200 is shown as including one first check valve assembly 180 extendinginto each separate coolant channel 160 of the passive containmentcooling system 200, it will be understood that, in some exampleembodiments, the passive containment cooling system 200 may includemultiple first check valve assemblies 180 that each extend from thecontainment environment 192, through the thickness 141 of thecontainment structure 140, into the same coolant channel 160, at a sameor different depths below the bottom 120 b of the coolant reservoir 120within the coolant channel 160.

Referring now to FIG. 3 , in some example embodiments, the passivecontainment cooling system 200 may include, in addition to the firstcheck valve assembly 180, one or more additional, or second check valveassemblies 380 at a position that is a vertical depth DB380 below abottom 120 b of the coolant reservoir 120 and thus at a vertical depthDT380 below a top surface 122 t of coolant fluid 122 in the coolantreservoir 120, where the one or more second check valve assemblies 380is in fluid communication with both the coolant reservoir 120 and withthe containment environment 192. As shown in FIG. 3 , a second checkvalve assembly 380 may extend through the thickness 141 of thecontainment structure 240 and into the coolant channel 160 at a verticaldepth DB380 below a bottom 120 b of the coolant reservoir 120, and thusat a vertical depth DT380 below the top surface 122 t of the coolantfluid 122 within the coolant reservoir 120, but example embodiments arenot limited thereto and the outlet 380 o of the second check valveassembly 380 may be open to another, separate conduit other than anycoolant channel 160 at vertical depth DB380/DT380, where the other,separate conduit is in fluid communication with the coolant reservoir120 and thus establishes fluid communication between the second checkvalve assembly 380 and the coolant reservoir 120. It will be understoodthat the vertical depth DT380 is equal to a sum of the vertical depthDB380 and the coolant reservoir depth D122 of coolant fluid 122, fromthe bottom 120 b to the top surface 122 t, in the coolant reservoir 120.As shown, the vertical depth DB380 may be less than the vertical depthDB180. For example, where the first and second check valve assemblies180 and 380 both extend through the containment structure 140 to coolantchannel 160 the one or more second check valve assemblies 380 may belocated vertically higher in the coolant channel 160, and thus closer tothe coolant reservoir 120, than the first check valve assembly 180.

In some example embodiments, the second check valve assembly 380includes an inlet conduit 381 i that is open to the containmentenvironment 192 via inlet 380 i, and outlet conduit 3810 that is influid communication with the coolant reservoir 120 at vertical depthDB380/DT380 (e.g., is open to the coolant channel 160 or any otherconduit to the coolant reservoir 120 at depth DB380/DT380), and one ormore check valves 382 coupled between the inlet conduit 381 i and theoutlet conduit 381 o. It will be understood that the configuration ofconduits and check valves 382 in the second check valve assembly 380 maybe any of the configurations that the first check valve assembly 180 mayhave, including any of the configurations shown in any of FIGS. 2A-2C,such that the second check valve assembly 380 may include any seriesconnection and/or parallel connection of check valves 382 that may beincluded in the first check valve assembly 180, and the configuration ofcheck valves 382 in the second check valve assembly 380 may be the sameas, or different than, the configuration of check valves 182 in thefirst check valve assembly 180.

Similarly to the first check valve assembly 180, the second check valveassembly 380 is configured to selectively open a flow conduit 387, andthus selectively enable one-way flow 398 of the containment fluid 197,to the coolant reservoir from the containment environment 192, based onthe one or more check valves 382 of the second check valve assembly 380actuating to open in response to a pressure at the inlet(s) 382 i of theone or more check valves 382, and thus the pressure P192-3 of thecontainment environment 192 at the second check valve assembly inlet 380i, and thus the pressure P192-3 in the containment environment 192 atvertical depth DB380/DT380 (where pressure P192-3 may be the same as ordifferent than the pressure P192-1 at any given time) being equal to orgreater than (e.g., reaching) a second threshold magnitude PX2. Thesecond threshold magnitude PX2 may be different than the first thresholdmagnitude PX1. The second threshold magnitude PX2 may at least partiallycorrespond to a hydrostatic pressure P380 of the coolant fluid 125 inthe coolant channel 160 at the outlet 380 o of the second check valveassembly. Restated, the second threshold magnitude PX2 may at leastpartially correspond to the hydrostatic pressure P380 of the coolantfluid 125 at depth DT380 below the top surface 122 t of the coolantfluid 122 in the coolant reservoir 120, and thus may correspond to(e.g., equal to or be greater than by a particular proportional marginand/or margin magnitude) the pressure head of coolant fluid 122 at depthDT380 of coolant fluid. Similarly to the first threshold magnitude PX1,in some example embodiments the second threshold magnitude PX2 maycorrespond to (e.g., match or exceed by a particular margin proportionor magnitude) a reference hydrostatic pressure P380 at depth DT380 thatresults from the coolant reservoir 120 being filled with coolant fluidto the particular reference depth D122.

It will be understood that the one or more check valves 382 of thesecond check valve assembly 380 may operate in the same manner describedherein with reference to the one or more check valves 182 of the firstcheck valve assembly 180 and thus may be configured to provide passiveventing of the containment environment 192.

Because the second check valve assembly 380 is spaced vertically abovethe first check valve assembly 180 in the passive containment coolingsystem 200 by a vertical spacing distance dH380, and because in someexample embodiments pressures P192-3 and P192-1 may be the samemagnitude at the same time (e.g., when the containment environment 192is filled with gas at least between depths DB180/DT180 and DB380/DT380),the one or more check valves 382 of the second check valve assembly 380may actuate to open and selectively enable one-way flow 398 from thecontainment environment 192 to the coolant channel 160 via the secondcheck valve assembly 380 when pressure P192-3/P192-1 is equal to asecond threshold magnitude PX2 that is greater than the hydrostaticpressure P380 at depth DT380 but is less than the hydrostatic pressureP180 at depth DT180. The one or more check valves 182 of the first checkvalve assembly 180 may subsequently actuate to open to selectivelyenable one-way flow 198 in response to pressure P192-3/P192-1subsequently increasing from the second threshold magnitude PX2 to thefirst threshold magnitude PX1. It will be understood that the first andsecond check valve assemblies 180 and 380 may independently actuate toindependently selectively enable or inhibit respective one-way flows 198and 398 of containment fluid 197, and thus the “venting” of containmentfluid 197 provided by the passive containment cooling system 200 may beprovided at an incremental rate that is proportional to the pressure inthe containment environment 192, as more flow conduits 387, 187 may beestablished by more check valve assemblies 380, 180 as the pressurewithin the containment environment 192 rises. The quantity of open flowconduits 187, 387 may be increased or reduced as the pressure within thecontainment environment 192 rises or falls, respectively, and suchproportional and independent opening and closing of flow conduits may beimplemented without (e.g., independently of) any operator interventionand thus may be understood to be a passive proportional ventingcapability provided by the passive containment cooling system 200.

While FIG. 3 illustrates only a single second check valve assembly 380,it will be understood that the passive containment cooling system 200may include any quantity of second check valve assemblies 380 that maybe located at same or different vertical heights in the coolant channel160 and may have separate, respective threshold pressures PX based onthe respective depths of the respective second check valve assembliesbelow the bottom 120 b of the coolant reservoir 120. It will beunderstood that in some example embodiments the passive containmentcooling system 200 may not include any second check valve assemblies380.

While FIG. 3 illustrates a second check valve assembly 380 extendinginto a coolant channel 160, it will be understood that exampleembodiments are not limited thereto. For example, a second check valveassembly 380 of the passive containment cooling system 200 may, insteadof extending into a coolant channel 160, be routed to the coolantreservoir 120 via one or more other, separate conduits, also referred toas separate pathways or parallel pathways, into which the one or moresecond check valve assemblies 380 may extend. For example, a secondcheck valve assembly 380 may extend, from the containment environment192, into a separate conduit, also referred to as a separate pathway orparallel pathway, (not shown in FIG. 1 or 3 ) that may extend to thecoolant reservoir 120 independently of the one or more coolant channels160. Accordingly, in some example embodiments, one or more second checkvalve assemblies 380 may be configured to enable “venting” of one ormore one-way flows 398 of containment fluid 197 to the coolant reservoir120 independently of the one or more coolant channels 160, therebyenabling the coolant reservoir 120 to retain at least some of thematerial of the coolant fluid 197, independently of the one or morecoolant channel 160. In some example embodiments, a first check valveassembly 180 may extend into a coolant channel 160 while a second checkvalve assembly 380 extends into a separate conduit that extends to thecoolant reservoir 120 independently of the coolant channel 160 intowhich the first check valve assembly 180 extends, or any other coolantchannel 160. In some example embodiments, a second check valve assembly380 may extend into a coolant channel 160 while a first check valveassembly 180 extends into a separate conduit that extends to the coolantreservoir 120 independently of the coolant channel 160 into which thesecond check valve assembly 380 extends, or any other coolant channel160.

It will be understood that, in some example embodiments, the first checkvalve assembly 180 may be absent from some or all of the coolantchannels 160. In some example embodiments, the passive containmentcooling system 200 may not include any first check valve assemblies 180.

Still referring to FIG. 1 , and further referring to FIG. 5 , thepassive containment cooling system 200 may include a fusible plug 190 ata bottom vertical depth DB190 below a bottom 120 b of the coolantreservoir 120, and thus a depth DT190 below the top surface 122 t of thecoolant fluid 122 in the coolant reservoir 120, where the fusible plug190 is in fluid communication with the coolant reservoir 120 and withthe containment environment 192. For example, as shown in FIG. 5 , thefusible plug 190 may extend, between opposite ends 190 i, 190 o, throughthe thickness 141 of the containment structure 140 and into the coolantchannel 160 at a bottom vertical depth DB190 below a bottom 120 b of thecoolant reservoir 120, and thus a depth DT190 below the top surface 122t of the coolant fluid 122 in the coolant reservoir 120, but exampleembodiments are not limited thereto. For example, the end 190 o of thefusible plug 190 may be open to another, separate conduit other than anycoolant channel 160 at vertical depth DB190/DT190, where the other,separate conduit is in fluid communication with the coolant reservoir120 and thus establishes fluid communication between the fusible plug190 and the coolant reservoir 120. It will be understood that thevertical depth DT190 is equal to a sum of the vertical depth DB190 andthe coolant reservoir depth D122 of coolant fluid 122, from the bottom120 b to the top surface 122 t, in the coolant reservoir 120. The bottomvertical depth DB190/DT190 may be greater than the first vertical depthDB180/DT180 by a distance dH192, as shown in FIG. 1 , such that ahydrostatic pressure P190 of the coolant fluid 124/125 in the coolantchannel 160 at the bottom vertical depth DB190/DT190 (which may be ahydrostatic pressure P190 that corresponds to the pressure head ofcoolant fluid of depth DT190 of coolant fluid) is greater than thehydrostatic pressure P180 of the coolant fluid 125 in the coolantchannel 160 at the first check valve assembly outlet 180 o (e.g.,hydrostatic pressure P180 that corresponds to the pressure head of depthDT180 of coolant fluid).

In some example embodiments, the fusible plug 190 is configured to atleast partially melt in response to a temperature T192 in thecontainment environment 192 at the fusible plug 190 (e.g., at the end190 i of the fusible plug 190 that is open to the containmentenvironment 192) at least meeting a threshold temperature TX, such thatthe fusible plug 190 exposes a flow conduit 195 extending, betweenopposite ends 190 o and 190 i, between the coolant channel 160 or otherpathway to the coolant reservoir 120 at the bottom vertical depthDB190/DT190 into the containment environment 192 to at least partiallyflood the containment environment 192 with at least some of the coolantfluid 124, 125. As shown in FIGS. 1 and 5 , the fusible plug 190 may bepositioned at the bottom of the coolant channel 160, e.g., at height H4,such that the coolant fluid 124 that passes over the end 190 o of thefusible plug 190 that is open to the coolant channel 160, and thus wouldbe the coolant fluid that would flood the containment environment 192 inresponse to the fusible plug 190 at least partially melting, would bethe colder, coolant fluid 124 and would thus provide improved coolingwithin the containment environment 192. The fusible plug 190 may be anywell-known type of fusible plug, including a fusible plug that includesa cylindrical body 191 (e.g., comprising brass, steel, etc.) extendingthrough the thickness 141 of the containment structure 140 and having aninner surface 191 i defining an inner cylindrical conduit 195 (alsoreferred to herein as a flow conduit, a fluid conduit, or the like) thatis filled with a fusible alloy 193 (e.g., tin) that is configured tomelt in response to a temperature T192 at the end 190 i of the fusibleplug 190 reaching a threshold temperature TX (e.g., the melting point ofthe fusible alloy 193) such that the fusible alloy 193 may at leastpartially melt to open (e.g., expose) the cylindrical conduit 195extending through the cylindrical body 191 and thus to establish a flowconduit through the fusible plug 190, via the exposed conduit 195, andthus to enable coolant fluid 124 to flow through the conduit 195 andinto the containment environment 192. Once introduced into thecontainment environment 192, the flooding coolant fluid 124 may providecooling of the containment environment 192 and/or nuclear reactor 100,containment, cooling, and control of radioactive materials in thecontainment environment (e.g., FCM, LFCM, corium, any combinationthereof, o the like), reduce pressure in the containment environment 192(e.g., via cooling and condensing steam in the containment environment192), any combination thereof, or the like.

In some example embodiments, the first check valve assembly 180 isconfigured to, based on the one or more check valves 182 selectivelyopening in response to the pressure P192-1 in the containmentenvironment 192 at the first check valve assembly inlet 180 i beingequal to or greater than the first threshold magnitude PX1, maintain apressure P192-2 in the containment environment 192 at the bottomvertical depth DB190/DT190 at a magnitude that is less than thehydrostatic pressure P190 of the coolant fluid 124 in the coolantchannel 160 at the bottom vertical depth (e.g., DB190, and thus DT190),to enable flow of coolant fluid 124 through the exposed conduit 195 ofthe fusible plug 190 and into the containment environment 192 inresponse to the fusible plug 190 at least partially melting. Forexample, the first check valve assembly 180 may be vertically spacedapart from the fusible plug 190 by a vertical distance dH192, and theone or more check valves 182 may be configured to actuate to an openstate in response to the pressure at the inlets 182 i of the one or morecheck valves 182 reaching a threshold pressure PX1 that is less than thehydrostatic pressure P190 in the coolant channel 160 at the depthDB190/DT190 such that 1) the one or more check valves 182 open beforethe pressure P192-2 reaches the magnitude of the hydrostatic pressureP190, thereby ensuring that P192-2 does not reach the magnitude ofhydrostatic pressure P190 and thus a pressure gradient from the coolantchannel 160 to the containment environment 192 through the fusible plug190 is ensured (thereby mitigating or preventing backflow out of thecontainment environment 192 through the fusible plug 190, and 2) apressure gradient is present from depths DB190/DT190 to DB180/DT180within the containment environment 192 when the fusible plug 190 atleast partially melts (after the one or more check valves 182 haveopened), so that a flow of fluid through the containment environment 192proceeds from the fusible plug 190 to the first check valve assemblyinlet 180 i. It will be understood that, in some example embodiments,pressure P192-2 in the containment environment 192 at depth DB190/DT190may be the same as, or different than, pressure P192-1 at the inlet 180i of the first check valve assembly 180.

In some example embodiments, the first check valve assembly 180 isconfigured to selectively enable the one-way flow 198, based on the oneor more check valves 182 actuating to open, in response to pressureP192-1 reaching a threshold magnitude PX1 that is lower than a pressuremagnitude that corresponds to the temperature T192 at end 190 i of thefusible plug 190 reaching the threshold temperature magnitude TX. Forexample, the fusible plug 190 may be configured to at least partiallymelt when temperature T192, at pressure P192-2, is a particularthreshold temperature TX, and the temperature T192 may correspond to themagnitude of pressure P192-2, and the one or more check valves 182 maybe configured to actuate to open in response to an inlet-side pressure(e.g., pressure at inlet 182 i) being at a first threshold magnitude PX1that is less than the pressure that corresponds to temperature T192being the threshold temperature TX. Accordingly, the first check valveassembly 180 may be configured to ensure that the one or more checkvalves 182 are open, and thus the flow conduit 187 is open and one-wayflow 198 is enabled, when the temperature T192 reaches the thresholdtemperature TX and the fusible plug begins to at least partially melt,such that venting is ensured to be ongoing when the fusible plug 190 atleast partially melts to expose conduit 195. Accordingly, the passivecontainment cooling system 200 may be configured to ensure that conduit187 is open when conduit 195 is exposed, thereby establishing a conduitinto the containment environment 192 via conduit 195 and out of thecontainment environment 192 via conduit 187.

In some example embodiments, the first check valve assembly 180 and thefusible plug 190 are collectively configured to enable circulation ofcoolant fluid 124 within the containment environment 192, from thecoolant channel 160 to the containment environment 192 via the exposedconduit 195 through the fusible plug 190 at the bottom vertical depthDB190/DT190 and from the containment environment 192 to the coolantchannel 160 via the first check valve assembly 180 at the first verticaldepth DB180/DT180. Accordingly, coolant fluid may circulate in and outof the containment environment 192 in an upwards flow direction thatensures that colder coolant fluid 124 enters the containment environment192 via the melted fusible plug 190 flow conduit 195 and replaces heatedcoolant fluid within the containment environment 192, and the heatedcoolant fluid in the containment environment 192 is removed from thecontainment environment 192 via the first check valve assembly 180 to bereturned to the coolant reservoir 120 to retain any entrainedradioactive materials and thus to at least temporarily retain saidmaterials within the nuclear plant 1, thereby improving containment.

It will be understood that multiple fusible plugs 190 may extend throughthe thickness 141 of the containment structure 140, from the containmentenvironment 192, to a same, common coolant channel 160, at a same ordifferent depths from the bottom 120 b of the coolant reservoir 120within the coolant channel 160.

While FIG. 1 illustrates one or more fusible plugs 190 extending intoone or more coolant channels 160, it will be understood that exampleembodiments are not limited thereto. For example, one or more fusibleplugs 190 of the passive containment cooling system 200 may, instead ofextending into a coolant channel 160, be routed to the coolant reservoir120 via one or more other, separate conduits, also referred to asseparate pathways or parallel pathways, into which the fusible plug 190may extend. For example, a fusible plug 190 may extend, from thecontainment environment 192, into a separate conduit, also referred toas a separate pathway or parallel pathway, (not shown in FIG. 1 or FIG.5 ) that may extend to the coolant reservoir 120 independently of theone or more coolant channels 160. Accordingly, in some exampleembodiments, one or more fusible plugs 190 may be configured to enableat least partial flooding of the containment environment 192 via coolantfluid that is supplied to the fusible plug 190 via a pathway from thecoolant reservoir 120 that is separate and independent of the one ormore coolant channels 160 of the passive containment cooling system 200.It will be understood that, in some example embodiments, a fusible plug190 may extend into a conduit, or pathway, to the coolant reservoir 120that is independent of (e.g., coupled to the coolant reservoir 120 inparallel with) a conduit, pathway, or coolant channel 160 into which afirst check valve assembly 180 and/or second check valve assembly 380may extend.

It will be understood that, in some example embodiments, the fusibleplugs 190 may be absent from some or all of the coolant channels 160. Insome example embodiments, the passive containment cooling system 200 maynot include any fusible plugs 190.

FIG. 4 is a flowchart that illustrates a method of operation of apassive containment cooling system, according to some exampleembodiments. The method shown in FIG. 4 may be performed with regard toany of the example embodiments of passive containment cooling system 200as described herein, including any of the example embodiments shown inFIGS. 1, 2A-2C, 3, and 5-6 .

As shown in FIG. 4 , the method may include cooling operations 401,check valve assembly operations 411, and fusible plug operations 421.Operations 401, 411, and 421 may be performed at least partiallyconcurrently (e.g., simultaneously), sequentially, or the like. In someexample embodiments, operation 411 may be performed independently ofoperations 401 and 421. In some example embodiments, operation 421 maybe performed independently of operations 401 and 411. In some exampleembodiments, operations 411 and/or 421 may be omitted such thatoperation 401 is performed alone. In some example embodiments, operation421 may be performed in response to the first check valve assembly 180opening the flow conduit 187 and selectively enabling the one-way flow198 in operation 411, as the passive containment cooling system 200 maybe configured such that a fusible plug 190 of the passive containmentcooling system 200 at least partially melts when the temperature T192 isat a magnitude corresponding to a pressure P192-1 at which the one ormore check valves 182 of the first check valve assembly 180 are open.

It will be understood that, in some example embodiments, operation 421may be omitted, for example where the passive containment cooling system200 does not include any fusible plugs 190.

Referring first to operation 401, At S402, the method may includedirecting a coolant fluid 124 to flow downwards from a coolant reservoir120 via a coolant supply conduit 150, according to gravity, to a coolantchannel 160 coupled to the containment structure 140 that at leastpartially defines the containment environment 192 for a nuclear reactor100, wherein the coolant channel 160 extends vertically along thecontainment structure 140, such that the coolant fluid 124 is directedinto a bottom of the coolant channel 160 according to gravity.

At S404, the coolant fluid 124 in the coolant channel 160 absorbs heat102 rejected by the nuclear reactor 100 in the containment environment192 via at least the containment structure 140. Such coolant fluid 124that absorbs the heat 102 becomes a heated coolant fluid 125 andexperiences a change in buoyancy (e.g., an increased buoyancy) anddensity (e.g., a decreased density) in relation to the buoyancy anddensity of the colder coolant fluid 124 that is supplied to the bottomof the coolant channel 160.

At S406, the heated coolant fluid 125 rises (e.g., flows upwards)through the coolant channel 160 from the bottom of the coolant channel160 toward the coolant reservoir 120 via a top of the coolant channel160 according to the change in heated coolant fluid 125 buoyancy, inrelation to coolant fluid 124 buoyancy, resulting from the coolant fluid125 absorbing heat 102 at S404. The rising heated coolant fluid 125 maybe displaced, at the bottom of the coolant channel 160, by fresh, coldercoolant fluid 124 via the coolant supply conduit 150.

At S408, the rising heated coolant fluid 125 reaches the top of thecoolant channel 160 and continues to rise, through the coolant returnconduit 170, as coolant fluid 126, according to the increased buoyancyand reduced density of the coolant fluid 126 over the coolant fluid 124that is being supplied into the bottom of the coolant channel 160. Thecoolant fluid 126 rises upwards, through the coolant return conduit 170,and thus, at S410, flows into the upper region 121 a of the coolantreservoir 120 via the outlet 174 of the coolant return conduit 170. Thecoolant fluid 126 may remain in the upper region 121 a based on havingincreased buoyancy and reduced density over the colder coolant fluid 123b in the lower region 121 b of the coolant reservoir 120. In someexample embodiments, the coolant fluid 126 in the coolant reservoir 120may cool over time and may sink down into the lower region 121 b ascoolant fluid 123 b, to thus be directed back to the bottom of thecoolant channel 160, thereby establishing a circulation of coolant fluidbetween the coolant reservoir 120 and the coolant channel 160.

In some example embodiments, the heat removed from the containmentenvironment 192 by the heated/return coolant fluid 125/126 may beretained in the coolant reservoir 120 for at least a period of time. AtS412, in some example embodiments, the removed heat may be furtherremoved from the coolant reservoir 120 via one or more various heatexchangers 128, thereby reducing or preventing the risk of heat removaldegradation or overheating of the passive containment cooling system200.

Referring to operation 411, concurrently with or separately from any ofS402 to S412 of operation 401, at S420 and S422, in response to thepressure at an inlet 182 i of any check valves 182 of a first checkvalve assembly 180 reaching a corresponding threshold pressure PX atwhich the respective check valve 182 is configured to actuate to an openstate (e.g., S420=YES), the check valve(s) 182 may open. For a checkvalve 182 having an inlet 182 i that is open to an inlet 180 i of thefirst check valve assembly 180, the pressure at the inlet 182 i of saidcheck valve 182 is the pressure P192-1 in the containment environment192 at the inlet 180 i, and thus the check valve 182 may actuate to theopen state (e.g., “open”) in response to the pressure of the containmentenvironment at the inlet 180 i of the first check valve assembly 180reaching the threshold pressure PX1 of the check valve 182. When allcheck valves 182 between an inlet 180 i and an outlet 180 o of a firstcheck valve assembly 180 are open, a flow conduit 187 is opened and aone-way flow 198 from the containment environment 192 to the coolantchannel 160 is selectively enabled, and thus, at S424, a containmentfluid 197 may flow from the containment environment 192 to the coolantchannel 160 via the one or more opened check valves 182 of the firstcheck valve assembly 180.

If, at S426, the pressure at the inlet 180 i (e.g., pressure P192-1)does not drop below the first threshold magnitude PX1 (e.g., S426=N0),the flow conduit 187 remains open and the one-way flow 198 through thefirst check valve assembly 180 is maintained. If, at S426 and S428, thepressure P192-1 drops below the threshold pressure (e.g., S426=YES), theone or more check valves 182 of the first check valve assembly 180 mayactuate to the closed state and thus the flow conduit 187 is closed andthe one-way flow 198 is inhibited. The one-way flow 198 may besubsequently re-enabled if, at S420 and S422, the pressure P192-1subsequently rises back to at least the threshold pressure PX1.

It will be understood that the above operations S420-S428 of operation411 may be performed in parallel with any of the operations S400 to S412of operation 401. The above operations S420-S428 of operation 411 aredescribed above with reference to the first check valve assembly 180,but it will be understood that, where the passive containment coolingsystem 200 includes one or more second check valve assemblies 380 inaddition to the first check valve assembly 180, operations S420-S428 maybe performed in parallel with regard to the one or more second checkvalve assemblies 380, in parallel with operations S420-S428 beingperformed with regard to the first check valve assembly 180.

Concurrently with or separately from any of S402 to S412 and/or S420 toS428 (e.g., operation 401 and/or operation 411), at S430 and S432, oneor more fusible plugs 190 at a bottom vertical depth DT190 in thecoolant channel 160 may at least partially melt (e.g., based on thefusible alloy 193 extending through a conduit 195 defined by acylindrical body 191 between an end 190 i that is open to thecontainment environment 192 and an opposite end 190 o that is open tothe coolant channel 160), based on a temperature T192 at the containmentenvironment-facing end 190 i of the fusible plug 190 reaching athreshold temperature TX (e.g., S430=YES), where the thresholdtemperature TX may be a melting temperature of the fusible alloy 193 atthe pressure P192-2 of the containment environment 192 at the end 190 i.As a result of said at least partial melting at S432, at least some ofthe coolant fluid 124 in the coolant channel 160 at depth DT190 may, atS434 and as shown by line 422, flow through the conduit 195 exposed as aresult of the melting at S432 into the containment environment 192thereby at least partially flooding the containment environment 192.

At S436, the coolant fluid 124 flooding the containment environment 192may, if the containment environment 192 is filled with coolant fluid upto depth DT180, rise to depth DT180 based on absorbing heat from thecontainment environment 192, and the coolant fluid may, as shown by line431, flow through the open flow conduit 187 through check valve assembly180 at depth DT180, as part of the one-way flow 198, back into thecoolant channel 160 at depth DT180 to be returned to the coolantreservoir 120 in S406 to S410. In some example embodiments, S420=YES andS426=NO whenever operation S430=YES, such that the flow conduit 187 maybe open whenever the fusible plug 190 at least partially melts at S432.

While a number of example embodiments have been disclosed herein, itshould be understood that other variations may be possible. Suchvariations are not to be regarded as a departure from the spirit andscope of the present disclosure, and all such modifications as would beobvious to one skilled in the art are intended to be included within thescope of the following claims. In addition, while processes have beendisclosed herein, it should be understood that the described elements ofthe processes may be implemented in different orders, using differentselections of elements, some combination thereof, etc. For example, someexample embodiments of the disclosed processes may be implemented usingfewer elements than that of the illustrated and described processes, andsome example embodiments of the disclosed processes may be implementedusing more elements than that of the illustrated and describedprocesses.

The invention claimed is:
 1. A method for operating a passivecontainment cooling system for a nuclear reactor, the method comprising:directing a flow of a coolant fluid downwards out of a lower region of acoolant reservoir via a coolant supply conduit according to gravity to abottom of a coolant channel that extends vertically along a containmentstructure that at least partially defines a containment environment inwhich the nuclear reactor is located; causing the coolant fluid to risethrough the coolant channel from the bottom of the coolant channeltoward an upper region of the coolant reservoir via a top of the coolantchannel according to a change in buoyancy of the coolant fluid based onthe coolant fluid absorbing heat rejected from the nuclear reactor inthe containment environment via at least the containment structure; andselectively enabling a one-way flow of a containment fluid, from thecontainment environment to the coolant reservoir through the coolantchannel via a first check valve assembly and the coolant channel, thefirst check valve assembly at a first vertical depth below a top surfaceof the coolant fluid in the coolant reservoir, the first check valveassembly in fluid communication with the coolant reservoir through thecoolant channel and in fluid communication with the containmentenvironment, wherein the selectively enabling is based on one or morecheck valves of the first check valve assembly opening in response to apressure at an inlet of the one or more check valves being equal to orgreater than a first threshold magnitude, the first threshold magnitudeat least partially corresponding to a hydrostatic pressure of thecoolant fluid at an outlet of the first check valve assembly at thefirst vertical depth.
 2. The method of claim 1, wherein the firstthreshold magnitude is greater than a reference hydrostatic pressure ofthe coolant fluid at the first vertical depth below the top surface ofthe coolant fluid in the coolant reservoir that results from the coolantreservoir being filled to a reference reservoir depth.
 3. The method ofclaim 1, further comprising: inhibiting the one-way flow, subsequentlyto selectively enabling the one-way flow, based on the one or more checkvalves closing in response to the pressure of the containmentenvironment at an inlet of the first check valve assembly being lessthan the first threshold magnitude.
 4. The method of claim 1, whereinthe one or more check valves include a series connection of a pluralityof check valves between an inlet of the first check valve assembly andthe outlet of the first check valve assembly, each check valve of theplurality of check valves is configured to open in response to apressure at an inlet of the check valve being equal to or greater thanthe first threshold magnitude, and the selectively enabling is based onall check valves of the series connection of the plurality of checkvalves opening.
 5. The method of claim 1, wherein the one or more checkvalves include a parallel connection of a plurality of sets of one ormore check valves between an inlet of the first check valve assembly andone or more check valve assembly outlets, each check valve of theplurality of sets of one or more check valves is configured to open inresponse to a pressure at an inlet of the check valve being equal to orgreater than the first threshold magnitude, and the selectively enablingis based on any set of one or more check valves of the parallelconnection of the plurality of sets of one or more check valves.
 6. Themethod of claim 1, wherein the selectively enabling is based on a burstdisc coupled in series with the inlet of the one or more check valvesand an inlet of the first check valve assembly rupturing in response toa pressure at the inlet of the first check valve assembly at the firstvertical depth being equal to or greater than the first thresholdmagnitude.
 7. The method of claim 1, further comprising: directing atleast a portion of the coolant fluid at a bottom vertical depth belowthe top surface of the coolant fluid in the coolant reservoir to flowinto the containment environment via an exposed flow conduit between thecoolant reservoir and the containment environment through the coolantchannel at the bottom vertical depth to at least partially flood thecontainment environment, based on a fusible plug in fluid communicationwith the coolant reservoir through the coolant channel and with thecontainment environment at the bottom vertical depth, at least partiallymelting to expose the exposed flow conduit in response to a temperaturein the containment environment at an end of the fusible plug that isopen to the containment environment being equal to or greater than athreshold temperature.
 8. The method of claim 7, wherein the first checkvalve assembly, based on selectively enabling the one-way flow,maintains a pressure in the containment environment at the bottomvertical depth at a magnitude that is less than the hydrostatic pressureof the coolant fluid at the bottom vertical depth, to enable flow of thecoolant fluid through the exposed flow conduit and into the containmentenvironment in response to the fusible plug at least partially melting.